Laser Linewidth Research Articles

Watt-level long-term stable ultra-narrow linewidth 1064 nm single-longitudinal-mode ring cavity fiber oscillator

This study presents a high-power, single-longitudinal-mode (SLM) fiber oscillator with a ring cavity design, operating at 1064 nm. Utilizing a double-cladding ytterbium-doped fiber as the gain medium, the system incorporates a fiber Bragg grating Fabry-Perot cavity and a dual coupler ring for step-by-step filtering to achieve SLM operation. With a pump power of 4.19 W, the oscillator delivered an output power of 1.01 W and narrowed the linewidth to 147 Hz, with the potential for further power increase. The oscillator demonstrated excellent longitudinal-mode stability, maintaining mode-hop-free operation for 8.7 hours after temperature stabilization. To the best of our knowledge, this work represents the longest duration of mode-hop-free operation and the narrowest laser linewidth achieved by a free-running ring-cavity SLM fiber oscillator at watt-level output powers.

Measurement of laser linewidth by delayed self-heterodyne interferometry with ultra-short optical fiber

Measurement of laser linewidth by delayed self-heterodyne interferometry with ultra-short optical fiber

Efficient single-frequency Tm:YAG crystal-derived silica fiber laser at 1.7 µm

A single-frequency distributed Bragg reflector (DBR) fiber laser operating at 1720 nm has been demonstrated for the first time using a Tm:YAG crystal-derived silica fiber (TCDSF), to the best of our knowledge. A single-frequency laser with an over 220 mW output power was achieved from a 1.5-cm-long TCDSF when being in-band pumped by a homemade 1610 nm fiber laser. The slope efficiency reached 25.30% for the absorbed pump power. The optical signal-to-noise ratio (OSNR) of the laser was ∼69.78 dB, and the laser linewidth was ∼84.5 kHz. Additionally, the measured relative intensity noise (RIN) remained at −130 dB/Hz at frequencies above 9 MHz. The experimental results indicate that the TCDSF is a promising gain medium for single-frequency lasers at 1.7 µm.

Chromatic dispersion-aware Tx and LO laser phase noise independent estimation enabled by dual pilot tones in DSCM systems

In the backbone networks employing high modulation formats such as 64QAM and metropolitan area networks, data center interconnection networks using the distributed feedback (DFB) lasers with several megahertz laser linewidths dramatically degrades system performance in the equalization enhanced phase noise (EEPN). Even though digital subcarrier multiplexing (DSCM) systems are known for their tolerance to EEPN, they still cause performance degradation in this case. Therefore, the EEPN mitigation algorithm becomes an indispensable part of DSP algorithms in DSCM systems. A dual pilot tone-based chromatic dispersion-aware transmitter and local oscillator laser phase noise independent estimation algorithm is proposed in this paper to solve the above problem. Simulations/offline experiment results under 5 MHz laser linewidth and 1000/800 km transmission indicate that the proposed algorithm could bring about 1/0.6 dB improvement of the required SNR for the normalized generalized mutual information of 0.9 compared with the blind phase searching algorithm.

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.

Demonstration of an ultra-wideband wavelength self-adaptive and high-precision single-frequency laser linewidth measurement system

Demonstration of an ultra-wideband wavelength self-adaptive and high-precision single-frequency laser linewidth measurement system

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.

Theoretical analysis for estimating laser integrated linewidth via frequency-noise power spectral density.

The measurement of a laser linewidth is significant in metrology, coherent optical communications, high-resolution sensing, and LIDAR. Firstly, in this study, we theoretically explain why estimating an integrated linewidth via a frequency-noise power spectral density (PSD) is valid. We find that the previous methods estimating the integrated linewidth via the frequency-noise PSD result from Gaussian approximation and obtain a more general consequence. Secondly, according to the theory, we propose the Voigt approximation method to improve the estimation performance. The simulation results show the Voigt approximation estimation error is lower than 5%. Finally, based on the Voigt approximation, the relationship between the interference visibility and laser linewidth is found, providing a possible convenient approach to measuring the linewidth.

145 W single-frequency fiber laser at 1607 nm with the seed and MOPA configuration

In this paper, high Er3+ concentration erbium doped fiber (EDF) and erbium-ytterbium co-doped fiber (EYDF) were fabricated for the seed and master oscillator power amplifier (MOPA) system of the single-frequency fiber laser. An in-band pumping source with the wavelength of 1535 nm was proposed to improve the efficiency in the ring-cavity. A slope efficiency of 23.8% with an output power of 49 mW was achieved and the laser linewidth was measured to be 800 Hz. In the MOPA system, a piece of EYDF was used firstly to absorb the backward 1 µm amplified spontaneous emission (ASE) and amplified the signal from the seed. Based on the above methods, an output power of 145 W was achieved at 1607.75 nm. Because of the self-phase modulation, the laser linewidth was expanded to be 10.75 kHz at the output power of 132 W. To the best of our knowledge, this is the highest efficiency and output power of the seed and the highest output power of the 1.6 µm single-frequency fiber laser.

Enhancing stimulated Brillouin scattering in suspended silicon waveguides through subwavelength nanostructuration [Invited

Brillouin optomechanics is playing a key role in the development of groundbreaking devices and novel functionalities in integrated silicon photonics, such as narrow linewidth filtering and lasers, tunable frequency, non-reciprocity, etc. Most silicon-based optomechanical waveguides, which use anchoring arms or perforated slabs to ensure mechanical stability and operate for transverse-electric polarized light, face challenges with acoustic mode leakage into the lateral Si slab, limiting the photon-phonon overlap and the Brillouin gain. Here, we propose new waveguide designs based on subwavelength nanostructuration to tailor near-infrared photons and GHz phonons and maximize the Brillouin gain. We introduce six different geometries suitable for both membrane or fully suspended configurations (i.e., without transversal arms anchoring the core to the Si slab). Our three-dimensional optomechanical simulations predict that subwavelength silicon membranes with strip, slot, and SWG slot core waveguides achieve gains up to 12257 W-1m-1 at mechanical frequencies of 12-13 GHz. Moreover, suspended silicon waveguides with SWG slots achieve a high gain of 43542 W-1m-1 at 4.45 GHz, with the ability to adjust the mechanical frequency from 4 to 9 GHz. Further enhancements in the Brillouin gain are studied by integrating side arms to amplify the moving boundaries effect in the suspended SWG slot waveguides and leveraging the slow light regime, which can significantly increase the Brillouin gain up to 17 × 106 W-1m-1 for a mechanical mode at 11.18 GHz.

Reduction of speckle effect using galvanometer scanners in a laser-based uniform source for radiance responsivity measurement

Narrow linewidth lasers play an important role in high-spectral and high-precision radiometric calibration. In this paper, a standard uniform light source for radiometric calibration of imaging detectors and devices is designed and realized based on narrow linewidth laser and intergrating sphere. Because the speckle effect caused by laser has a great influence on the radiance transfer, galvanometer scanners were employed to reduce the speckle effect. CCD method was used to measure the nonuniformity of the middle row/column of the integrating sphere exit, and nonuniformity reduced from 2.6 % to 0.72 % when the galvanometer scanners were turned on. By superimposing 9 different regions of static speckle image, the non-uniformity is greatly reduced, which is consistent with the experimental results. The output radiance stability of the source is measured with an improvement from 8.4 × 10-5 to 2.7 × 10-5. Finally, the nonuniformity of the source is measured to be 0.15 % by a spot luminance meter.

Improvement of optical noise in optical-injection-locked quantum dot lasers epitaxially grown on silicon by reducing external carrier noise

Abstract This study theoretically investigates the impact of external carrier noise from pumping sources on the optical noise of epitaxial quantum dot (QD) lasers on silicon. The findings indicate that the spectral linewidth and relative intensity noise (RIN) of silicon-based QD lasers using a quiet pump are significantly reduced. At 5.5 times the threshold current, the spectral linewidth decreases from 337.2 kHz to 213.2 kHz, and the RIN decreases from −141.3 dB Hz−1 to −168.8 dB Hz−1. This reduction is attributed to the lower external carrier noise level of the quiet pump, which also suppresses the spectral linewidth rebroadening effect at a high bias current. Moreover, when external optical injection locking is applied, the spectral linewidth further decreases to 24 kHz at an injection ratio of −70 dB and to 1.8 mHz at 0 dB. The RIN also slightly decreases to −172.4 dB Hz−1 with an injection ratio of 10. These results demonstrate that using a quiet pump is an effective and manageable strategy for significantly reducing both the spectral linewidth and RIN of QD lasers, thereby facilitating their application in next-generation photonic integrated circuits, continuous-variable quantum key distribution, quantum computing and ultra-precise quantum sensing.

An 8-Channel Narrow Linewidth Multi-Wavelength Laser Hybrid Integrated by Photonic Wire Bonding Structures for DWDM Systems

An 8-channel monolithically integrated narrow linewidth multi-wavelength laser array (NL-MLA) combined with the commercial 8×1 planar lightwave circuit (PLC) coupler by the photonic wire bonding (PWB) technique is demonstrated in this paper. A distributed feedback (DFB) laser with the high-reflection and anti-reflection coatings (HR-AR) and the tapered asymmetric corrugation-pitch-modulated (TACPM) grating structure is proposed. The TACPM grating is achieved by the reconstruction equivalent chirp (REC) technique, which is low-cost and of high wavelength accuracy. According to the numerical simulation results, under the unavoidable impact of the facet phase of the HR end, the TACPM DFB laser suppresses the longitudinal spatial hole burning (LSHB) effect significantly, while having better single-mode performance and control of wavelength compared with the conventional HR-AR DFB laser. An 8-channel HR-AR TACPM DFB NL-MLA is fabricated with the 0.8 nm wavelength spacing design. The hybrid integration between the 8-channel NL-MLA and the PLC chip is achieved using the PWB technique. Both good single-mode performance and accurate wavelength interval of 0.8 nm can be obtained when all channels work simultaneously. The laser linewidth is below 273.8 kHz with good uniformity, and the relative intensity noise is under -159.6 dB/Hz for each channel.

Brillouin laser pumped tunable low-threshold mid-IR Kerr comb at 2 μm

Optical frequency combs in the 2 μm wavelength region are important for applications ranging from sensing of gases such as CO2 and CO to optical communications, LIDAR, and gravitational wave detection. The development of low-loss waveguides and high-Q microresonators with anomalous dispersion and the availability of tunable narrow linewidth lasers around 1.55 μm have enabled the realization of small footprint soliton combs and low-threshold Kerr combs in this wavelength region; demonstrations of microresonator frequency combs in the 2 μm wavelength region have been limited. Here, we harness an intracavity pumping scheme to demonstrate a low-threshold (<100 mW) microresonator Kerr comb at 2 μm. We exploit Brillouin lasing in a silica microsphere (∼310 μm diameter) to create an intracavity pump, which then generates a ∼140 nm wide Kerr comb in the backscattered Stokes direction. We demonstrate the tolerance of the comb generation scheme to microsphere dimensions and the input pump wavelength by achieving Kerr comb generation in microspheres of diameters ranging from 295 to 318 μm and also at different input pump wavelengths for a particular microsphere diameter. Intracavity pumping opens up opportunities for the development of soliton combs and Kerr combs in the mid-IR wavelength region for applications such as dual-comb spectroscopy, LIDAR, and optical communications.

Bias-aided DD-LMS equalizer for optical multipath-interference-impaired high-speed PAM4 transmission systems

Multipath interference (MPI) noise induces drastic fluctuations in high-speed 4-level pulse amplitude modulation (PAM4) intensity modulation direct detection (IMDD) systems, severely degrading the transmission performance. Here, we propose a bias-aided decision-directed least mean square (DD-LMS) equalizer to eliminate the MPI noise. In the simulation, the proposed bias-aided DD-LMS equalizer could adeptly track and compensate the MPI-impaired PAM4 signal, markedly improving the bit error rate (BER) performance under different MPI levels and laser linewidths. Within a 112 Gbps PAM4 transmission system, the proposed equalizer achieves a MPI tolerance over 4 dB at the KP4-forward error correction (FEC) threshold (2.4 × 10−4), outperforming existing MPI suppression techniques.

Dual-media laser system: Nitrogen vacancy diamond and red semiconductor laser.

Diamond is a potential host material for laser applications due to its exceptional thermal properties, ultrawide bandgap, and color centers, which promise gain across the visible spectrum. More recently, coherent laser methods offer improved sensitivity for magnetometry. However, diamond fabrication is difficult in comparison to other crystalline matrices, and many optical loss channels are not yet understood. Here, we demonstrate a continuous-wave laser threshold as a function of the pump intensity on nitrogen-vacancy (NV) color centers. To achieve this, we constructed a laser cavity with both an NV diamond medium and an intracavity antireflection-coated diode laser. This dual-medium approach compensates intrinsic losses of the cavity by providing a fixed additional gain below threshold of the diode laser. We observe a continuous-wave laser threshold of the laser system and linewidth narrowing with increasing green pump power on the NV centers. Our results are a major development toward coherent approaches to magnetometry.

Controllable lasing characteristics in Brillouin random fiber lasers by tailoring gain and random feedback fibers

Brillouin random fiber lasers (BRFLs), the combination of stimulated Brillouin scattering (SBS) gain and random distributed feedback, offer narrow linewidth lasing with simplicity and flexibility. However, BRFLs may not gain broad acceptance unless the fundamental lasing mechanisms governing their operation are fully understood and the lasing properties are effectively manipulated. Here, we demonstrate the control of lasing characteristics in BRFLs by tailoring the SBS gain fiber and the scattering pattern of the random feedback fiber. Experimental results show that BRFLs with 5 cm random fiber gratings (RFGs) feedback exhibit lower intensity fluctuation, longer coherence time, and more stable frequency compared to those with 6 km Rayleigh scattering fiber (RSF) feedback thanks to more correlated phase, lower mode density, and weaker dependence on external variations of RFGs. The low-randomness RFG feedback further improves the coherence time and intensity fluctuation attributed to the small period variation of sub-gratings. Moreover, the BRFL based on the high gain fiber and the strong scattering RFG feedback with low loss achieves high lasing efficiency and low threshold. The frequency jitter, intensity noise, and coherence time are also improved by reducing the gain fiber from 20 km to 1 km due to decreased mode hopping from mode competition. These results clarify the impact of gain and random feedback fibers on BRFL performance, offering insights for optimizing complex laser design for applications requiring high frequency stability and long coherence time.

Eight-wavelength-switchable single-frequency fiber laser with scattering-enhanced fiber-based wavelength self-adaptive linewidth compression

We demonstrate an eight-wavelength-switchable single-frequency (SF) ultra-narrow linewidth erbium-doped fiber laser (EDFL). A switchable polarization-dependent eight-channel filter (S-PDECF), fabricated using femtosecond laser direct-writing technology, determines the eight wavelengths of the laser output, with four channels concentrated near 1530 nm and the other four near 1550 nm. A dual-coupler ring-based compound-cavity filter is utilized to achieve single-longitudinal-mode selection for the EDFL. The introduction of a femtosecond laser direct-written scattering-enhanced fiber (SEF) effectively narrows the laser linewidth, enabling ultra-narrow linewidth outputs lasing at all eight wavelengths, and the wavelength self-adaptive linewidth compression of the SEF is verified for the first time. Furthermore, the incorporation of SEF significantly enhances the overall performance of the EDFL. The eight switchable single-wavelength laser outputs all exhibit exceptional characteristics, including spectral wavelength fluctuations of ≤0.011 nm, spectral peak power fluctuations of ≤0.54 dB, optical signal-to-noise ratios of ≥71 dB, linewidths of ≤248 Hz, output power fluctuations of ≤0.252 dB over a 10-min measurement period, and relative intensity noise of ≤−150.17 dB/Hz @≥ 3 MHz.

Magnetic resonance linewidths independent with the laser linewidth in coherent population trapping (CPT) magnetometers

Coherent population trapping (CPT) magnetometers are the magnetic measuring instrument that utilize the interference effect between light and atoms. It has a broad range of applications in weak magnetic measurement. However, current CPT magnetometers have low sensitivity, which does not satisfy the ultra-high sensitivity requirements of weak magnetic measurement. This paper is aimed to improve this problem by studying the relationship between the sensitivity of CPT magnetometer and the laser linewidth. It has been concluded that the magnetic resonance linewidth of the CPT system remains independent of the laser linewidth according to the Liouville equation, firstly. Furthermore, the correctness of this theory has been verified through design experiments. Additionally, we have identified certain issues that require attention in the miniaturization and chip-scale CPT magnetometer on vertical cavity surface emitting laser (VCSEL).

High-power diode laser spectrally narrowed with prism-etalon feedback.

A simple method for reducing the linewidth of a diode laser while maintaining high output power is described. It is based on a dispersive prism and a thin etalon for retroreflective feedback. The etalon creates two weak external cavities that provide spectral selectivity that is periodic with a period equal to the etalon's free spectral range. The method was applied to a multimode blue laser diode, which in the absence of feedback features a linewidth of several nanometers. The spectral properties of the laser were investigated for different etalon thicknesses and operating currents and tested in the presence of temperature fluctuations. With a SF11 equilateral uncoated prism near Brewster's angle and a 0.3 mm-thick uncoated fused silica etalon, the linewidth was reduced 20-fold to 70pm (3.6 cm-1) with an output power of 3W at a current of 2.15A. The largest diode current probed was 2.75A, which resulted in a linewidth of 100pm (5.1 cm-1) and an output power of 4W. In contrast to the use of, for example, a volume Bragg grating, a high degree of flexibility is afforded as the same prism-etalon pair can be used across the visible and near infrared.