Ultra-narrow Linewidth Laser Research Articles

Wideband microwave frequency response measurement based on optical phase-locked loop

Frequency response measurement, or the forward transmission coefficient (S21) measurement for a two-port network, is the key function of a vector network analyzer (VNA). In this paper, a broadband and high dynamic range (DR) microwave S21 parameter measurement scheme based on an optical phase-locked loop (OPLL) is proposed. By heterodyning two phase-locked hybrid integrated ultra-narrow linewidth lasers, a microwave signal with low phase noise and spurious level is generated as the incident signal and reference signal, and the signal frequency can be easily manipulated over a wide range by tuning the master laser wavelength. In the receiver, the radio frequency (RF) signals are down-converted to intermediate frequency (IF) signals with the phase-locked lasers. By sampling and processing the IF signals the S21 parameter of the DUT can be acquired. A proof-of-concept experiment is performed, and with available photodetectors, phase modulators and phase-locked loops, a measurable range of 2 to 18 GHz is achieved. The demonstrated minimum frequency resolution of the OPLL-based RF signal synthesizer is 10 Hz. The system DR exceeds 68 dB at an equivalent resolution bandwidth of 1 kHz. The S21 parameters of a power divider and a bandpass filter are measured, and the results are well consistent with those of a commercial VNA. The DR and measurable range limit factors and possible extension methods are discussed. The proposed approach offers a high potential way to develop a wideband, high DR, and fully integrated VNA.

C + L-band tunable sub-kHz linewidth single frequency fiber laser by combining a fiber sub-ring with a saturable absorber

A sub-kHz linewidth single-frequency fiber laser that can be tuned in the full range of C + L band is proposed and experimentally demonstrated. A broad band gain fiber by bidirectional pumping combining with a wide tunable filter is used to realize 75.87 nm tuning range from 1529.98 to 1605.85 nm, which can be further improved if a wider tunable filter is available. To achieve single-frequency ultra-narrow linewidth laser operation, a ring-cavity laser configuration with a fiber sub-ring and a saturable absorber is utilized. The measured optical signal-to-noise ratio (OSNR) of the fiber laser is over 80 dB, the side mode suppression ratio (SMSR) can be improved to ∼60 dB, and the linewidth can be narrowed to ∼453 Hz. The output power of the laser exceeds 28 mW, and the slope efficiency is 2.05 %. This proposed fiber laser has the advantages of narrow linewidth, excellent wavelength tunability, and high output power, which is promising for the applications in optical sensing and optical communication systems.

Laser frequency noise characterization using high-finesse plano-concave optical microresonators.

Characterizing laser frequency noise is essential for applications including optical sensing and coherent optical communications. Accurate measurement of ultra-narrow linewidth lasers over a wide frequency range using existing methods is still challenging. Here we present a method for characterizing the frequency noise of lasers using a high-finesse plano-concave optical microresonator (PCMR) acting as a frequency discriminator. To enable noise measurements at a wide range of laser frequencies, an array of PCMRs was produced with slight variations of thickness resulting in a series of discriminators operating at a series of periodical frequencies. This method enables measuring the frequency noise over a wide linewidth range (15 Hz to <100 MHz) over the 1440-1630 nm wavelength range. To assess the performance of the method, four different lasers were characterized, and the results were compared to the estimations of a commercial frequency noise analyzer.

Tunable ultra-narrow linewidth linear-cavity fiber lasers assisted by distributed external feedback

We proposed and demonstrated a widely tunable and ultra-narrow linewidth fiber laser scheme by exploiting an original Fabry-Perot laser main cavity and a distributed external feedback structure. This was feasible with the aid of a feedback signal with distributed characteristics which is injected into the main cavity to induce an intense mode competition effect. Eventually, a single frequency fiber laser with a side mode suppression ratio of 69 dB, an output linewidth of 570 Hz, a white frequency noise of 110 Hz2/Hz at the frequency offset that is beyond 2 × 105 Hz, and a relative intensity noise less than −140 dB/Hz is successfully obtained under normal conditions. Also, the high spectral purity performance of the laser can be maintained under different pump powers for each wavelength channel. This work presents a promising method to obtain tunable ultra-narrow linewidth lasers.

Ultra-narrow linewidth laser system based on intra-cavity electro-optic crystal frequency stabilization of external-cavity diode laser

An external-cavity diode laser based on an intra-cavity electro-optic modulator tuning is developed in this paper. Electro-optic crystal lithium niobate (LiNbO3), with no hysteresis effect and high response speed, is used to replace the traditional tunable part piezoelectric transducer (PZT). The linewidth of this laser at free-running mode is about 133 kHz. In addition, in the ultra-narrow linewidth laser system, the error signal is fed back to the driving voltage of LiNbO3 to realize laser locking, which can avoid the problem of laser output power fluctuation caused by feeding back to the laser injection current. The self-developed crystal-tuned laser has been successfully used in an ultra-narrow linewidth laser system, and the experimental results show that the linewidth of its beat frequency signal is about 1.5 Hz.

High-Resolution Slow-Light Fiber-Bragg-Grating Microphone and Hydrophone

This paper reports a slow-light fiber-Bragg-grating (FBG) pressure sensor that functions both as a microphone and a hydrophone and pushes the limit of acoustic pressure resolution of conventional FBG sensors operated without external diaphragms. The sensor utilizes a narrow slow-light resonance to enhance its optical response (greater transmission change per wavelength shift), a soft polymeric coating to enhance its acoustic response (greater wavelength shift per pressure), and an ultra-narrow linewidth laser to reduce its laser frequency noise (which often dominates in such sensors). The coated fiber microphone exhibits an average resolution of ~210 μPa/√Hz (100 Hz to 10 kHz), which is ~6 times better than the previous record. When used as a hydrophone, the resolution is ~880 μPa/√Hz (1 to 100 kHz), which is better than the previous record for a passive FBG hydrophone by a factor of ~23 over the same acoustic frequency range. The wavelength shift per unit pressure is larger for the coated FBG than for the bare FBG by a measured factor of ~16 in air and ~3.7 in water. This last value differs markedly from a published ~300-fold enhancement using a conventional FBG hydrophone coated with the same polymer. It is shown that most of this discrepancy is due to a referencing error in the publication that reported this improvement. With further improvement, slow-light FBGs could be utilized for high-resolution applications such as in the deep sea and seismic research.

Research on Brownian Thermal Noise Limit of a Cylindrical Ultra-Stable Cavity with Support Pads

Brownian thermal noise (TN) of ultra-stable cavities (USCs) imposes a fundamental limitation on the frequency stability of ultra-narrow linewidth lasers. This work investigates the TN in cylindrical USCs with the four support pads in detail through theoretical estimation and simulation. To evaluate the performance of state-of-the-art ultra-narrow linewidth lasers, we derive an expression of the TN for a cylindrical spacer according to the fluctuation–dissipation theorem, which takes into account the front face area of the spacer. This estimation is more suitable for the TN of the cylindrical USC than the previous one. Meanwhile, we perform detailed studies of the influence of the four support pads on the TN in cylindrical USCs for the first time by numerical simulations. For a 400 mm long cylindrical USC with an ultra-low expansion spacer and fused silica substrates, the displacement noise contributed from the four support pads is roughly four times that of the substrates and the GaAs/AlGaAs crystalline coating. The results show that the four support pads are the primary TN contributors under some materials and geometries of USCs.

Tailoring the pulse train of an optical frequency comb with a magnetized atomic medium

Selecting specific modes from the complete pulse train of optical frequency comb (OFC) is helpful in ultra-narrow linewidth laser preparation. Here, the mode selector for a commercial OFC based on the magnetized atomic medium is demonstrated in both theory and experiment. Only the comb modes with specific frequencies experience polarization rotation because of the Faraday effect, thus the mode tailoring can be realized by eliminating unchanged OFC modes via polarization selection. The theory study demonstrates the rich tunability of this device, and three specifically chosen operating conditions validate the system’s characteristics. The selected pulse group is distinguished in both optical and frequency spectrum, where spectral differences and bandwidth can be observed accordingly. Further, the selected pulse group and a continuous-wavelength laser stimulate an electromagnetically induced transparency spectrum in the hyperfine Paschen–Back regime to show the pulse group characteristics in detail. The method proposed provides the experimental foundation for producing narrow linewidth lasers from a polychromatic source with tailored results retaining the excellent characteristics of the original.

Cooling of an integrated Brillouin laser below the thermal limit.

Photonically integrated resonators are promising as a platform for enabling ultranarrow linewidth lasers in a compact form factor. Owing to their small size, these integrated resonators suffer from thermal noise that limits the frequency stability of the optical mode to ∼100 kHz. Here, we demonstrate an integrated stimulated Brillouin scattering (SBS) laser based on a large mode-volume annulus resonator that realizes an ultranarrow thermal-noise-limited linewidth of 270 Hz. In practice, yet narrower linewidths are required before integrated lasers can be truly useful for applications such as optical atomic clocks, quantum computing, gravitational wave detection, and precision spectroscopy. To this end, we employ a thermorefractive noise suppression technique utilizing an auxiliary laser to reduce our SBS laser linewidth to 70 Hz. This demonstration showcases the possibility of stabilizing the thermal motion of even the narrowest linewidth chip lasers to below 100 Hz, thereby opening the door to making integrated microresonators practical for the most demanding future scientific endeavors.

Narrow linewidth self-injection locked fiber laser based on a crystalline resonator in add-drop configuration.

Strong and narrow spectral feedback plays a key role in self-injection locking (SIL) single-frequency lasers, especially in the stabilization of single longitudinal mode (SLM) operation of a multi-mode laser. Here, we report on a narrow linewidth SIL fiber laser that adopts a fiber add-drop configuration composing of two tapered fibers and a high-Q MgF2 crystalline whispering-gallery-mode resonator. The feedback from the drop port could be controlled and optimized for stable SLM lasing of multi-mode fiber lasers. A stable single-frequency fiber laser with white frequency noise as low as ∼0.4 Hz2/Hz, corresponding to an instantaneous linewidth of ∼1.26 Hz, is demonstrated. Compact, controllable, and all-fiber configuration in this work to achieve an ultra-narrow linewidth laser will attract interest in many applications.

422 Million intrinsic quality factor planar integrated all-waveguide resonator with sub-MHz linewidth

High quality-factor (Q) optical resonators are a key component for ultra-narrow linewidth lasers, frequency stabilization, precision spectroscopy and quantum applications. Integration in a photonic waveguide platform is key to reducing cost, size, power and sensitivity to environmental disturbances. However, to date, the Q of all-waveguide resonators has been relegated to below 260 Million. Here, we report a Si3N4 resonator with 422 Million intrinsic and 3.4 Billion absorption-limited Qs. The resonator has 453 kHz intrinsic, 906 kHz loaded, and 57 kHz absorption-limited linewidths and the corresponding 0.060 dB m−1 loss is the lowest reported to date for waveguides with deposited oxide upper cladding. These results are achieved through a careful reduction of scattering and absorption losses that we simulate, quantify and correlate to measurements. This advancement in waveguide resonator technology paves the way to all-waveguide Billion Q cavities for applications including nonlinear optics, atomic clocks, quantum photonics and high-capacity fiber communications.

Spontaneous formation of a macroscopically extended coherent state

It is a straightforward result of electromagnetism that dipole oscillators radiate more strongly when they are synchronized, and that if there are $N$ dipoles, the overall emitted intensity scales with $N^2$. In atomic physics, such an enhanced radiative property appears when coherence among two-level identical atoms is established, and is well-known as "superradiance" \cite{Dicke:1954aa}. In superfluorescence (SF), atomic coherence develops via a self-organisation process stemming from the common radiated field, starting from a incoherently prepared population inversion \cite{Bonifacio:1975aa}. First demonstrated in a gas \cite{Skribanowitz:1973} and later in condensed matter systems \cite{Florian:1984}, its potential is currently being investigated in the fields of ultranarrow linewidth laser development for fundamental tests in physics \cite{Meiser:2009,Meiser:2010,Bohnet:2012aa,Norcia:2016, Norcia:2018}, and for the development of devices enabling entangled multi-photon quantum light sources \cite{Raino:2018aa,Angerer:2018aa}. A barely developed aspect in superradiance is related to the properties of the dipole array that generates the pulsed radiation field. In this work we establish the experimental conditions for formation of a macroscopic dipole via superfluorescence, involving the remarkable number of $4\times10^{12}$ atoms. Even though rapidly evolving in time, it represents a flexible test-bed in quantum optics. Self-driven atom dynamics, without the mediation of cavity QED nor quantum dots or quantum well structures, is observed in a cryogenically-cooled rare-earth doped material. We present clear evidence of a decay rate that is enhanced by more than 1-million times compared to that of independently emitting atoms. We thoroughly resolve the dynamics by directly measuring the intensity of the emitted radiation as a function of time.

Distributed time delay sensing in a random fiber grating array based on chirped pulse φ-OTDR.

A high-precision distributed time delay measurement in a chirped pulse phase optical time domain reflectometry (CP φ-OTDR) system based on a random fiber grating array is proposed and demonstrated, in which a temperature-induced refractive index and fiber dimension change associated time delay could be measured for distributed temperature sensing. The random fiber grating array includes many inscribed refractive index change locations at periods of sub-micron. When laser pulses are launched into the fiber grating, the backscattered light possesses many unique localized speckle patterns at different locations. These patterns change with temperature, and hence the backscattering spectral response will change accordingly. By measuring the localized speckle pattern change due to the change of the temperature over the chirped pulse spectrum and performing cross-correlation calculation, we can realize distributed temperature measurements in real time using a megahertz bandwidth distributed feedback laser. Unlike a conventional φ-OTDR sensing system which measures a distributed phase change along the fiber using an ultra-narrow linewidth laser, the distributed time delay presented in this Letter is directly measured in real time. It is shown that the time-resolved localized pattern trace is stable with very small fluctuation, thanks to the enhanced inhomogeneity and reflectivity. The minimum detectable temperature variation is about 0.028°C at meter order of magnitude spatial resolution.

Tunable passively Q-switched ultranarrow linewidth erbium-doped fiber laser

A tunable passively Q-switched erbium-doped fiber laser with ultranarrow linewidth is proposed and demonstrated. With the single wall carbon nanotubes (SWCNTs) as the passive Q-switch, the ultranarrow linewidth laser operation is realized based on spectral filtering effect.using an ultranarrow tunable bandpass filter (UNTBF). The Q-switched laser spectrum is tunable from 1525 to 1561 nm, covering a wavelength range of 36 nm. At 1545 nm, the highest repetition rate of ~38 kHz and the lowest pulse width of 1.15 μs are obtained, with the corresponding pulse energy of approximately 0.18nJ. The linewidth measurement using self-heterodyne technique yields a linewidth value of 17.5 kHz, which may be the narrowest linewidth for a Q-switched fiber laser to the best of our knowledge.

Active loss compensation in the resonant optical gyroscope with a "reflector": towards a cost-effective alternative for the navigational grade.

The performance of passive fiber optic gyroscopes involving ring resonators is limited mainly by the loss and finesse of the cavity. In this work, we show performance enhancement of the recently studied resonant fiber optic gyroscope with a "reflector" using active loss compensation. Our gyroscope does not require expensive ultra-narrow linewidth lasers, expensive lock-in detection methods, or polarization-maintaining fibers, which are mandatory for all standard resonant fiber optic gyroscopes. The performance of this gyroscope shows four-fold enhancement in the $Q$Q factor $ (2.2 \times {10^8}) $(2.2×108), compared to an earlier experimental setup involving losses. Enhanced sensitivity to rotation is experimentally demonstrated using loss compensation as well as tuning the embedded reflector in the resonator. A shot-noise-limited sensitivity of 0.03 deg/h is possible with this experimental realization. This work demonstrates that our gyroscope can provide a cost-effective alternative, even in the navigational grade.

Effects of weak fiber Bragg gratings on a distributed vibration sensing system based on phase-sensitive optical time-domain reflectometry

A distributed fiber sensing system based on weak fiber Bragg gratings (wFBGs) enhanced phase-sensitive optical time-domain reflectometry (Φ-OTDR) has improved signal-to-noise ratio (SNR) and sensitivity. The effects of wFBGs on a Φ-OTDR system are investigated theoretically and experimentally. The SNR improvement induced by wFBGs, which have higher reflectivity compared with the Rayleigh scattering, has been analyzed in systems with different laser source linewidths. The experimental results show that Φ-OTDR assisted by wFBGs with −35-dB reflectivity has a lower noise level and its SNR is improved by 27 dB. By comparing laser sources with different linewidths, it is proved that the enhanced system no longer requires an ultranarrow linewidth laser. The SNR in different locations of a 2.4-km-wFBG-array remains at a high level.

Ultra-narrow linewidth laser based on a semiconductor gain chip and extended Si3N4 Bragg grating.

We demonstrate ultra-narrow linewidth fixed wavelength hybrid lasers composing a semiconductor gain chip and extended silicon nitride Bragg grating. Fabricated ultra-low κ Bragg gratings provide a narrow bandwidth and high side-lobe suppression ratio. A single-wavelength 1544nm hybrid extended-distributed Bragg reflector laser with 24mW output power and a Lorentzian linewidth of 320Hz is demonstrated, providing a high-performance light source for on- and off-chip applications.

Quasicritical coupling in a few-mode tapered-fiber coupled whispering-gallery-mode system

A quasicritical coupling state was demonstrated both in theory and experiment with a few-mode tapered-fiber (TF) coupled whispering-gallery-mode (WGM) cavity. In such a configuration, the coupling mode to activate the WGM and the drop mode are in the same TF and of almost the same decay rate with the variation of the gap between the TF and WGM cavity. Consequently, when the gap decreases to zero, the transmission of the coupling mode decreases monotonously to zero at the resonance wavelength, while the transmission of the drop mode increases monotonously to unit, which means that neither the coupling efficiency in the coupling mode nor the transmission of the drop mode will decrease in the traditional overcoupling region. Thus a steadily high coupling efficiency can be achieved accordingly, which we defined as quasicritical coupling. Such a coupling state will be helpful to develop WGM-cavity-based filters and benefit the fabrication of ultranarrow-linewidth lasers.

Optically pumped rare-earth-doped Al2O3 distributed-feedback lasers on silicon [Invited

This paper reviews recent progress in the field of optically pumped rare-earth-doped channel waveguide lasers, with a focus on operation utilizing distributed-feedback resonators on silicon wafers. Rare-earth-doped amorphous aluminum oxide thin films have been deposited onto silicon wafers by RF reactive co-sputtering from metallic Al and rare-earth targets, the spectroscopy and optical gain of Er3+, Yb3+, Nd3+, and Tm3+ ions has been investigated, and the near-infrared laser transitions near 1 μm in Yb3+, 1.5 μm in Er3+, and 2 μm in Tm3+ and Ho3+ have been demonstrated. Output power between a few μW and hundreds of mW have been achieved in different waveguide geometries, and ultranarrow-linewidth laser operation has been demonstrated.

Phase noise cancellation in polarisation-maintaining fibre links.

The distribution of ultra-narrow linewidth laser radiation is an integral part of many challenging metrological applications. Changes in the optical pathlength induced by environmental disturbances compromise the stability and accuracy of optical fibre networks distributing the laser light and call for active phase noise cancellation. Here we present a laboratory scale optical (at 578 nm) fibre network featuring all polarisation maintaining fibres in a setup with low optical powers available and tracking voltage-controlled oscillators implemented. The stability and accuracy of this system reach performance levels below 1 × 10-19 after 10 000 s of averaging.