Laser Linewidth Research Articles

Quantum noise and its evasion in feedback oscillators

Feedback oscillators, consisting of an amplifier whose output is partially fed back to its input, provide stable references for standardization and synchronization. Notably, the laser is such an oscillator whose performance can be limited by quantum fluctuations. The resulting frequency instability, quantified by the Schawlow-Townes formula, sets a limit to laser linewidth. Here, we show that the Schawlow-Townes formula applies universally to feedback oscillators beyond lasers. This is because it arises from quantum noise added by the amplifier and out-coupler in the feedback loop. Tracing the precise origin of quantum noise in an oscillator informs techniques to systematically evade it: we show how squeezing and entanglement can enable sub-Schawlow-Townes linewidth feedback oscillators. Our analysis clarifies the quantum limits to the stability of feedback oscillators in general, derives a standard quantum limit (SQL) for all such devices, and quantifies the efficacy of quantum strategies in realizing sub-SQL oscillators.

3.38 kW all-fiberized narrow linewidth fiber laser based on active polarization control using RMS-Prop algorithm

Active polarization control in high power narrow linewidth non-polarization maintained (non-PM) fiber amplifier is investigated in this paper. The non-PM fiber laser is realized employing bidirectional pump configuration seeded with narrow linewidth laser source. Root-Mean-Square propagation (RMS-prop) algorithm is developed and first employed in polarization control technique. When control system does not work, the polarization extinction ratio (PER) of output signal laser varies between 0.25 dB and 4.61 dB. When control system works, the PER stably remains above 12.2 dB during power amplification. At laser power of 3.38 kW, the PER is 12.2 dB, 3 dB linewidth is measured to be ∼0.32 nm, and beam quality (M2 factor) is M2x = 1.39, M2y = 1.50. This is the first time to realize over 3 kW linearly polarized fiber laser based on active polarization control to the best of our knowledge.

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.

Fabrication of a Counter-Directional Pump/Signal Combiner with Built-In Mode Field Adapter

This article introduces a novel counter-directional pump/signal combiner with a built-in mode field adapter. This combiner offers additional functionality to address the problem of mode field mismatch between the gain fiber and delivery fiber, which occurs when using a large-core (50 μm) signal fiber to suppress stimulated Raman scattering. The combiner exhibits negligible degradation in beam quality as a result of implementing mode field matching and employing an effective feedback alignment technique. The beam quality of the mode field adapter integrated in the combiner is improved by 10%, compared to the conventional approach, thanks to the reduction in splicing points and the prevention of fusion loss caused by tapering-induced cladding diameter mismatch. The combiner demonstrates an average pump coupling efficiency of 98.5% and a temperature rise coefficient of less than 10 °C/kW without active cooling. Furthermore, an integrated device based on the combiner is fabricated to shorten the length of delivery fiber, therefore mitigating the effects of stimulated Raman scattering. All those techniques mentioned above are utilized in a narrow linewidth laser system, resulting in an output of approximately 5840 W with a nearly single-mode characteristic.

Stable tunable narrow-linewidth fiber laser based on random distributed feedback

Stable narrow linewidth fiber lasers with relatively high output power have broad application prospects in optical communication, sensing and precision measurement, due to their high coherence and low noise characteristics, which are the main requirements to increase the detection range and accuracy of these systems. Obtaining narrow linewidth laser output with improved performance has been one key topic in the field of laser science and technology. Here, we propose a combination of transmission and reflection of randomly distributed Bragg grating arrays to compress the linewidth of laser emission in an Erbium-doped fiber system at the 1550 nm band, thereby providing a selective reflection-transmission channel effect without too many feedback points. A stable narrow linewidth fiber laser with a linewidth of 570 Hz (output power of 6 mW) and 830 Hz (output power amplified to 25 mW), a maximum signal-to-noise ratio (SNR) greater than 63 dB, and a wavelength adjustable in the range of 445 pm is realized.

L-Band Erbium Fiber Laser with Tunable and Narrow Sub-kHz Linewidth Output

In this paper, a wavelength-selectable erbium-doped fiber (EDF) laser in L-band bandwidth with a simple structure and low cost is described. To obtain stable single-longitudinal-mode (SLM) and narrow laser linewidth, the 2 m unpumped EDF-based saturable absorber (SA) and 100 m single-mode fiber (SMF)-induced Rayleigh backscattering (RB) injection effect are cascaded in the ring cavity of the fiber laser. The EDF SA and feedback RB injection can cause the auto-tracking ultra-narrow bandwidth filter effect to suppress the multiple longitudinal mode oscillations and narrow the laser linewidth. Thus, the tunable SLM wavelength output of the EDF laser with sub-kHz linewidth can be achieved. In the measurement, two EDF ring lasers can be executed when the placement of EDF-based SA and SMF-induced RB are switched to experiment with different output characteristics. The experimental results show that the wavelength bandwidths of the two lasers are between 1558.0 and 1618.0 nm. Additionally, the optimal optical signal-to-noise ratio (OSNR), wavelength linewidth, output power, and stability of the presented EDF laser configuration are also achieved and discussed.

Joint mitigation of frequency offset and phase noise for coherent free-space optical communication in atmospheric turbulence

In this paper, taking the atmospheric turbulence into account, two low-complexity algorithms which can jointly compensate the frequency offset (FO) and phase noise (PN) caused by laser line-width and atmospheric turbulence were proposed in coherent free-space optical (FSO) communications with polarization division multiplexing QPSK (PDM-QPSK) scheme. By introducing phase-conjugated counterparts for training symbols or information symbols on one of two orthogonal polarization states, the FO and PN were sensed simultaneously for a transmitted block or for each symbol. Simulations and experiments were performed to evaluate the BER performance. The results both manifested significant bit-error-rate (BER) performance improvement.

Symmetrical dual-sideband oppositely chirped differential FMCW LiDAR.

A differential FMCW LiDAR for high-precision distance measurements of remote non-stationary targets is proposed and demonstrated experimentally. The required positive and negative symmetrically oppositely chirped laser beams are generated synchronously through a fixed-frequency laser by employing externally unified broadband optical phase modulation and symmetrical dual-sideband optical filtering. After coaxial transmission and reception, orthogonally polarized optical beat signals containing target distance and vector velocity data are de-chirped separately by optical in-phase and quadrature demodulations and then synchronously received by four-channel photoelectric balance detectors. After differential processing of the received beat signals and a fast Fourier transform, it is possible to implement real-time simultaneous range and vector velocity measurements. The inherent symmetrically oppositely chirped optical frequency make it possible to measure the target distance immune to the internal random phase noise introduced by the spectral linewidth of the frequency-swept laser and the external random phase noise introduced by atmospheric turbulence, speckle, and vibration. Meanwhile, the measurement of the target velocity is immune to the nonlinearity of the frequency-swept laser. These results encourage an approach to overcome the barriers of coherence length, nonlinearity, and external noise, and implement simultaneous real-time ranging and velocimetry of long-range, rapid-moving targets.

High power single-frequency 1112 nm laser by an insertable Nd:YAG/YAG bonded monolithic planar ring oscillator.

A high power single-frequency operation at 1112 nm with novel insertable monolithic planar ring oscillator based on a Nd:YAG/YAG bonded crystal is proposed. In a proof-of-principle experiment, a finely designed coating on the output surface is carried out to ensure single-wavelength oscillation at 1112 nm, together with a half-wave plate and a Tb3Ga5O12 crystal inserted in the open space of the bonded block to realize the unidirectional operation with power scalability. Consequently, the single-frequency laser delivers an output power of 3.9W at 1112.3 nm with a slope efficiency of 58.6% and an optical-to-optical efficiency of 17.7%. The power fluctuation is measured to be within ± 0.26% over 20 min, and the laser linewidth is estimated to be 4.15 MHz (Δλ = 0.017 pm).

Adaptive decision threshold for an optical multipath-interference-impaired short-reach 50-Gbps PAM4 transmission.

The short-reach optical transmission systems based on intensity modulation and direct detection (IM/DD) are gradually evolving into the networks with complex link topologies and connections, especially inside the data center. Multipath interference (MPI) introduces irregular fluctuations in the 4-level pulse amplitude modulation (PAM4) signals and therefore affects the transmission performance. This Letter proposes an adaptive decision threshold (ADT) scheme to dynamically update the decision threshold, which can track the signal fluctuations in real time and mitigate the impact of MPI noise on the transmitted PAM4 signals. Numerical simulation results present that the proposed ADT scheme can significantly improve the bit error rate (BER) performance of the MPI-impaired PAM4 transmission system considering different MPI levels and laser linewidths. In a 50-Gbps PAM4 transmission system, the ADT can improve the MPI tolerance by more than beyond 6 dB when the BER reaches the KP4-forward error correction (FEC) criterion (2.4 × 10-4), presenting a better denoising performance than the existing MPI-mitigation algorithms A1 and A2. Moreover, the ADT scheme offers a lower computation complexity compared with A1 and A2, making it more practical for implementation.

Single-longitudinal-mode narrow linewidth broadband tunable Brillouin random laser including dispersion compensation fibers

We propose and demonstrate a single-longitudinal mode (SLM) narrow linewidth broadband tunable Brillouin random laser, including two segment dispersion compensated fibers (DCFs), in which enough narrowband Brillouin gain provided by a 2-km-length DCF to overcome the cavity loss, and Rayleigh backscattering (RBS) provided by a 1-km-length DCF form a mirrorless open cavity to continuously compress the laser linewidth. Compared to ordinary single mode fibers, DCF has an enhanced stimulated Brillouin scattering gain coefficient and RBS coefficient, which is attributed to its small core diameter structure, making the laser more compact and stable. Stronger random feedback effectively lowers the laser threshold, and enough Brillouin gain for multiple amplifications in random mode makes SLM operations more accessible. The proposed laser is with a threshold of ∼11 mW and an optical signal-to-noise ratio (OSNR) of ∼55.75 dB, the 3-dB linewidth and linewidth compression ratio of the output laser reaches ∼550 Hz and ∼236, respectively. By changing the wavelength of the pump laser, the laser wavelength can be continuously tuned in a broad wavelength range, which is only limited by tunable range of Brillouin pump laser. The laser wavelength fluctuation of 0.02 nm and the OSNR fluctuation of about 3 dB in a 30-minute measure period, indicating that the laser is with a good stability.

Impact of external carrier noise on the linewidth enhancement factor of a quantum dot distributed feedback laser.

This paper demonstrates that the linewidth enhancement factor of quantum dot lasers is influenced by the external carrier transport issued from different external current sources. A model combining the rate equation and semi-classical carrier noise is used to investigate the different mechanisms leading to the above phenomenon in the context of a quantum dot distributed feedback laser. Meanwhile, the linewidth enhancement factor extracted from the optical phase modulation method shows dramatic differences when the quantum dot laser is driven by different noise-level pumps. Furthermore, the influence of external carrier noise on the frequency noise in the vicinity of the laser's threshold current directly affects the magnitude of the linewidth enhancement factor. Simulations also investigate how the external carrier transport impacts the frequency noise and the spectral linewidth of the QD laser. Overall, we believe that these results are of paramount importance for the development of on-chip integrated ultra-low noise oscillators producing light at or below the shot-noise level.

Single-frequency fiber laser of 315 mW at 1940 nm based on a Tm : YAG/Ho : YAG-co-derived silica fiber.

A 1940 nm single-frequency distributed Bragg reflector (DBR) fiber laser was demonstrated based on a Tm : YAG/Ho : YAG-co-derived silica fiber (THCDSF). The THCDSF, which had a core dopant concentration of 8.02 wt.% Tm2O3 and 1.18 wt.% Ho2O3, was prepared via the melt-in-tube (MIT) method using a Tm : YAG and a Ho : YAG as the precursor core and a silica tube as the cladding. Employing 1.8 cm of the THCDSF, we achieved a maximum single-frequency output power of 315 mW at 1940 nm when pumped by a 1610 nm fiber laser. The slope efficiency of the laser was 29.68% for the absorbed pump power. The laser linewidth was less than 23.65 kHz, and the relative intensity noise (RIN) stabilized at -145 dB/Hz after exceeding 4.8 MHz. To the best of our knowledge, this is the first demonstration of a single-frequency DBR laser with a Tm3+/Ho3+ fiber as the gain medium.

Phase encoded quantum key distribution up to 380 km in standard telecom grade fiber enabled by baseline error optimization

Phase encoding in quantum key distribution (QKD) enables long-distance information-theoretic secure communication in optical fibers. We present a novel theoretical model characterizing errors from various sources in practical phase encoding-based QKD systems, namely the laser linewidth, detector dark counts, and channel dispersion. This model provides optimized optical pulse parameters and less distortion in pulses, which eliminates system imperfections and leads to a reduced quantum bit error rate (QBER) for practical QKD scenario. This analysis is applicable to various fiber-based phase and time encoding protocols. In particular, we implement this to a differential phase shift (DPS) QKD scheme operating at a 2.5 GHz clock, which produces a secure key rate of 193 bits/s at a fiber length of 265 km and an unprecedented QBER < 1% up to 225 km length with standard telecom components. We show that by adjusting the quantum efficiency and dark count rates of detectors, proposed system can establish secure keys up to 380 km distance using standard telecom grade fiber with a QBER of 1.48%. Moreover, the system is compatible with existing optical fiber networks and capable of establishing a secure key exchange between two cities 432 km apart using ultra-low-loss (ULL) specialty fiber.

Study of the velocity-selection satellites present in the () electric quadrupole transitions in atomic rubidium

This article presents detailed experimental and theoretical studies of the satellite fluorescence lines observed when the , (, ) electric quadrupole transition (E2) is excited in a room temperature vapor of rubidium atoms. The initial state of this E2 transition is prepared by a narrow linewidth (9 MHz) laser locked to one of the dominant D2 hyperfine excitations for zero-velocity atoms. Effects due to the selection of different atomic velocity classes in this preparation step are responsible for the presence of several satellite lines that can be found in the fluorescence decay spectra of both 85Rb and 87Rb. Compared to the main lines, these satellites do not show a strong dependence on the relative linear polarization directions of the lasers used in the preparation and the electric quadrupole steps. The relative intensities of the satellites decrease as the intensity of the preparation laser increases. Results of a rate equation calculation that explicitly includes selection-of-velocity effects indicate that optical pumping plays a central role in determining the relative intensities and the polarization dependence of the satellite lines. When the preparation laser is locked to the low-F cyclic transition in 87Rb, six lines were found in addition to the lone electric quadrupole transition for zero velocity atoms. In this case, the calculated spectrum is necessary for the correct interpretation of the experimental results, clearly indicating that optical pumping and selection of velocities are responsible for these six additional electric quadrupole lines.

Multi-Aperture Coherent Digital Combining Based on Complex-Valued MIMO 2N × 2 Adaptive Equalizer for FSO Communication

Collecting light using multiple small apertures offers a lot of advantages over doing so using a single large aperture, including the effective mitigation of scintillation under strong turbulence, low implementation cost, and ease of scalability. However, the tricky of a multi-aperture system is to combine the multiple branches signals with static skew mismatch and with time-varying characteristics of gain, phase and state of polarization, especially for coherent digital combining of polarization multiplexing (PM) signals. In this paper, a complex-valued multiple-input multiple-output (MIMO) 2N×2 adaptive equalizer is designed to combine multi-aperture signals, and the tap coefficient of each branch is calculated based on the constant modulus algorithm (CMA). This technique relaxes hardware constraints on the time alignment of multi-aperture branch since the skew can be compensated electronically thanks to the proposed blind digital combining algorithm. In addition, laser linewidth within evaluation range smaller than 10 MHz has no effect on the performance of 2N×2 adaptive equalizer. Moreover, the feasibility of the proposed adaptive equalizer for moderate and strong turbulence suppression is verified by a 10-Gbps data rate PM-QPSK modulation offline two- and four-aperture simulated turbulence experiment.

Selectable and stable single-frequency fiber laser with flattened power range and sub-kHz linewidth output

To achieve stable and tunable single-longitudinal-mode (SLM) wavelength output, an erbium-doped fiber (EDF) laser with two fiber ring architecture and unpumped EDF based saturable absorber (SA) is demonstrated experimentally. The acquired optical signal to noise ratio (OSNR), wavelength linewidth, output stability and power of the fiber laser over an available tuning bandwidth of 1523.0 to 1569.0 nm are also implemented. Furthermore, this EDF laser configuration also can distribute the output power uniformly in the span of 1527.0 to 1569.0 nm within 0.53 dB power difference.

PDM-SHD NFDM transmission with envelope-detection feedback polarization tracking and optical injection locking.

Nonlinear frequency division multiplexing (NFDM) systems, especially the eigenvalue communications have the potential to overcome the nonlinear Shannon capacity limit. However, the baud rate of eigenvalue communications is typically restricted to a few GBaud, making it challenging to mitigate laser frequency impairments such as the phase noise and frequency offset (FO) using digital signal processing (DSP) algorithms in intradyne detections (IDs). Therefore, we introduce the polarization division multiplexing-self-homodyne detection (PDM-SHD) into the NFDM link, which could overcome the impact of phase noise and FO by transmitting a pilot carrier originating from the transmitter laser to the receiver through the orthogonal polarization state of signal. To separate the signal from the carrier at the receiver, a carrier to signal power ratio (CSPR) unrestricted adaptive polarization controlling strategy is proposed and implemented by exploiting the optical intensity fluctuation of the light in a particular polarization rather than its direct optical power as the feedback. Optical injection locking (OIL) is used subsequently to amplify optical power of pilot carrier and mitigate the impact of signal-signal beat interference (SSBI). Additionally, the effects of cross-polarization modulation (XPolM) and modulation instability (MI) in long haul transmission are explored and inhibited. The results show that the tolerable FO range is about 3.5 GHz, which is 17 times larger than the ID one. When 16-amplitude phase shift keying (APSK) or 64-APSK constellations are used, identical Q-factor performance can be obtained by using distributed feedback (DFB, ∼10 MHz) laser, external cavity laser (ECL, ∼100kHz), or fiber laser (FL, ∼100 Hz), respectively, which demonstrates that our proposed PDM-SHD eigenvalue communication structure is insensitive to the laser linewidth. Under the impact of cycle slip, the Q-factor difference of 16-APSK signal between the ECL-ID system and ECL-SHD system can be up to 8.73 dB after 1500 km transmission.

Sub-kHz-linewidth laser generation by self-injection locked distributed feedback fiber laser

We presented an integrated all-fiber sub-kHz-linewidth distributed feedback fiber laser (DFB-FL) assisted by self- injection locking. A π phase-shifted fiber Bragg grating (π-FBG) was employed to achieve single longitudinal mode operation, which was UV-inscribed in Er/Yb co-doped fiber with super-Gaussian apodization profile. The experiment results have shown that the apodized π-FBG based laser cavity has got the higher slope efficiency and the narrower linewidth, compared with the normal grating, which are consistent with the simulation one very well. To further compress the linewidth, we have employed the self-injection locking mechanism, which the linewidth of output laser could reach to around 350 Hz with 10-m delay fiber. After integrated encapsulation, the relative intensity noise (RIN) peak of the DFB-FL was reduced to below −125 dB/Hz. The beat frequency fluctuation was less than 15 MHz within 1000 s and power fluctuation was less than 1% in 7 h.

A Method for Extracting a Laser Center Line Based on an Improved Grayscale Center of Gravity Method: Application on the 3D Reconstruction of Battery Film Defects

Extraction of the laser fringe center line is a key step in the 3D reconstruction of linear structured light, the accuracy of which is directly related to the quality of the 3D model. A laser center line extraction method based on an improved gray center of gravity method is proposed to solve the problem of low extraction accuracy. Firstly, a smoothing method is used to eliminate the flat top of the laser line, and the Gaussian curve is adopted to fit the peak position of the curve. Then, the gray threshold is set to automatically extract the laser linewidth, and based on the window opening, the grayscale center of gravity method is improved to extract the coordinates of the center pixel for the second time. Finally, experiments show that the average absolute error of the improved laser line extraction method is 0.026 pixels, which is 2.3 times lower than the gray center of gravity method, 1.9 times lower than the curve fitting method, and the standard error can reach 0.005 pixels. Compared with the gray center of gravity method and the curve fitting method, the influence of gray value change on the center line extraction is more fully considered, and the center of the light strip can be extracted more accurately, achieving sub-pixel accuracy.