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We propose and demonstrate a technique to vastly reduce the linewidth of a commercially available semiconductor laser operating at 1550 nm by locking it to an all-fiber Mach-Zehnder interferometer (MZI) with a short path length imbalance. The long term linewidth was reduced from 20 kHz to the single and even sub Hz level. Very narrow linewidths can only be measured by beating two separate, nominally identical lasers. To this end, we employ two lasers configured in a unique, counter propagating configuration, both locked to two different fringes of the MZI. The beat between the two locked lasers proves the extremely narrow linewidth of each laser. In order to detect and compensate for the noise caused by mechanical and thermal fluctuations of the fibers, we use the beat signal as an input to a servo loop that feeds a piezo electric fiber stretcher stabilizing the MZI. [ABSTRACT FROM AUTHOR]

Ultra-stable lasers serve as the backbone for the most advanced optical metrology scientific experiments, enabling the ability to laser interferometry or atomic spectroscopy at the highest levels of precision, thanks to the rapid development of laser wavelength stabilization techniques based on optical or electrical feedback from an external reference such as absorption line or optical cavity. With the increasing interest in making these scientific systems portable and applying it outside of the laboratory, it still remains an open question as to how to realize a laser source that can reach the high wavelength stabilization and still remain sufficiently compact and portable for field use. Here, we describe a wavelength stabilization fiber laser with intracavity reference mechanism, by utilizing the beat note of dual modes inside the laser cavity as the laser wavelength drift reference signal. A laser wavelength drift less than 5 kHz and ADEV 10−13 level at integration time scales from 0.01s to 1000s is achieved, which is to-date the best results for the laser wavelength stabilization without the external frequency reference. The outcome of this work presents a new way to achieve an ultra-stable laser with a simpler, lower cost feature. It offers new insights to promote the highest precision optical metrology scientific application outside of the laboratory, and also gives the possibilities of applying the ultra-stable laser source into a wider range of industrial application. [ABSTRACT FROM AUTHOR]

A stabilized coherent narrow-linewidth Brillouin random fiber laser with low lasing noise is proposed and investigated experimentally, which employs stimulated Brillouin scattering and Rayleigh scattering in optical fibers to provide the gain and the random distributed feedback, respectively. The stabilized lasing output with suppressed noise is realized utilizing a simple structure of a double-coupler fiber ring resonator, which is composed of a section of single-mode fiber and two optical couplers with different coupling ratios. Compared with the existing free-running Brillouin random fiber lasers, much more stabilized laser output can be obtained as the double-coupler fiber ring resonator effectively acts as a narrow-bandwidth mode filter through the Vernier effect that constrains the dense random longitudinal modes competition within a small frequency range. In the experiment, a stable ultra-narrow linewidth of approximately 586.4 Hz is obtained from the laser output. Statistical analysis of mode hopping in the proposed laser indicates a great improvement in multi-mode resonances and frequency jitters with the imbedding of the ring resonator. In addition, measurement results of relative intensity noise and frequency noise of the proposed laser showcase a great suppression in mode-competition-induced lasing instability and a significant improvement in the noise level compared to the free-running Brillouin random fiber lasers. The proposed all-fiber-based stabilization technique makes a step forward for developing practically robust and reliable Brillouin random fiber lasers. Such a stabilized highly coherent narrow-linewidth laser source with low cost and low power consumption will find promising applications in high-resolution spectrometers, microwave photonics, and optical sensing. [ABSTRACT FROM AUTHOR]

The paper is focused on a systematic investigation of the circuits for active stabilization of the frequency offset between two semiconductorn ultra-accurate fiber optic time transfer systems. The frequency offset is increased up to 50 GHz, which can be achieved not only with relatively low-noise integrated tunable laser assemblies, but also with ordinary, telecom-grade, distributed feedback lasers. The paper starts by determining the general frequency accuracy and stability, required to keep the uncertainty contribution of the stabilization circuit at a negligible level (assumed here as 1 ps) compared to other contributions of the overall uncertainty of the link calibration. Next, the technical details of the essential building block of the system discussed, which is required to convert the high-frequency offset to lower frequencies to allow convenient frequency measurement, are analyzed. Experimental circuits built with commercially available millimeter wave integrated circuits were tested with the frequency offset complying with telecom dense wavelength division multiplex standards, equal to 12.5 GHz, 25 GHz and 50 GHz. It was found that such stabilization circuits can cause substantial systematic errors, which are related to lasers’ phase noise and the operation with low input optical powers (below −39 dBm in evaluated circuits). These effects were investigated experimentally in detail and countermeasures were proposed. [ABSTRACT FROM AUTHOR]

We demonstrate a novel, robust and compact fiber laser mode-locked by nonlinear polarization evolution (NPE) in polarization-maintaining (PM) fibers. The reflectivity of the artificial saturable absorber (SA) is analyzed to explain the mode-locking mechanism in the laser cavity. Experimentally, three linear laser schemes that feature repetition rates 94 MHz, 124 MHz and 133 MHz are systematically investigated. When the pump power is 1100 mW, the 124-MHz laser cavity delivers highly stable pulses with a single-pulse energy of 0.92 nJ. After the compression, the pulse duration obtained from the 124-MHz fiber laser is 250 fs, while the corresponding transform-limited pulse duration is 124 fs. The highest fundamental repetition rate that could be achieved in our experiment is 133 MHz, as mentioned above. The noise characterization has been performed with different cavity lengths and therefore different net-cavity dispersion. The 68-fs timing jitter and the 0.01% relative intensity noise (RIN) of the 133-MHz fiber laser have been realized integrated from 1 kHz to 10 MHz. Furthermore, the root-mean-square (RMS) power fluctuation is 0.35% in 2 hours, which implies superior stability of the output power. Thus, this linear fiber oscillator provides a competitive low-noise light source for optical applications appropriate for complex environments. [ABSTRACT FROM AUTHOR]

The closed-loop method based on the Faraday effect is realized with a low coherence-noise resonator in resonance fiber optic gyro (RFOG). The resonator based on spun-fiber (SF) is designed and fabricated, which not only has the advantage of low coherent optical noises, but also has an outstanding circularly state of polarization (CSOP) maintaining ability that lays the foundation for the realization of a closed-loop method based on the Faraday effect. As the core component of the Faraday-based method, the Faraday coil is reasonably designed to meet the requirements of the sufficient frequency shift and power stability. In addition, the interval control method that is conducive to the improvement of system stability is applied to control Faraday coil for the first time. The experimental results show that the method we proposed can effectively achieve high-reciprocity and linear equivalent frequency shift by Faraday coil. Also, the short-term and long-term stability of Faraday-based RFOG system have been improved compared with other closed-loop methods, which provides a simple and efficient method for RFOG closed-loop signal processing. [ABSTRACT FROM AUTHOR]

Operation stability is the main challenge that current existing saturable absorbers (SAs) are facing. In this paper, we report on two low chemical activity saturable absorbers, the tellurium dioxide and antimony trioxide thin film, for stable femtosecond pulse generation in 2 μm region. The nonlinear absorption of two oxides at 2 μm was firstly revealed and the extraordinary mode-locking performance based on two oxides was demonstrated. The phase noise and amplitude noise were also measured and analyzed, and the integrated RMS relative intensity noise was found to be 0.0196% and 0.0218% for TeO2 thin film and Sb2O3 thin film respectively. Great thermal stability and long-term stability were also confirmed. Heating the oxides coated area in atmosphere for hours and the lasers still could be mode-locked. Compared to low dimensional materials, the oxides proposed in this paper could be more suitable for robust mode locking in harsh environment. The broadened working conditions tolerance also enables new applications for optical modulators. [ABSTRACT FROM AUTHOR]

In this paper, we report the existence of Eckhaus instability in laser cavities with harmonically swept filters, of which Fourier Domain Mode Locked (FDML) laser is an important example. We show that such laser cavities can be modeled by a real Ginzburg Landau equation with a frequency shifting term arisen from the cavity dispersion. We analytically derived a solution of the governing equation and analyzed its stability. We found that the cavity dispersion introduces a continuous frequency shift to the laser signal such that it will be eventually pushed outside the stable region and trigger the Eckhaus instability. We show that the repeated triggering of the Eckhaus instability in the laser cavities is the dominant effect that leads to the high frequency fluctuations in FDML laser output, which is the unique feature of such laser cavities and intrinsically limits the signal quality of the FDML lasers with nonzero cavity dispersion. [ABSTRACT FROM AUTHOR]

Integrating photonics with CMOS electronics in silicon is essential to enable chip-scale, electronic-photonic systems that will revolutionize classical and quantum communication and computing systems. However, the lack of an on-silicon isolator, capable of blocking unwanted back reflections and ensuring the stable operation of the laser, precluded many previous demonstrations from providing single-chip solutions. For most optical systems employing a laser, magneto-optic isolators have been indispensable, but such isolators are incompatible with silicon. To stabilize on-chip lasers, reflections-cancellation circuits were proposed as a way to reduce the reflections going back to the laser. Yet, a stable laser against time-varying back reflections was never demonstrated. Here we demonstrate a stable quantum well-distributed feedback (QWDFB) laser against slowly time-varying reflections using a reflections-cancellation circuit (RCC) on a foundry-produced, silicon-photonic (SiP) chip. The optical spectrum and the relative intensity noise (RIN) of the laser when the RCC was running is comparable to when an isolator was used. By accurately locking the laser in a stable optical feedback regime, the RCC further enhances the QWDFB laser performance by reducing its linewidth by a factor of 100, down to 3 kHz. Both results are enabled using novel techniques in the design, calibration, tuning, and control of the proposed SiP RCC. The optical insertion loss of the RCC is less than 1.5 dB for reflections smaller than −20 dB and can yield isolation ranges of up to 64 dB. Our device paves the way towards the mass production of fully integrated, low-cost electronic-photonic silicon chips without attaching magneto-optic isolators between the laser and the SiP chip. [ABSTRACT FROM AUTHOR]

Random fiber lasers (RFLs), featuring a simple open cavity but with unique lasing characteristics, have captured considerable attention in the past several decades. Particularly, RFLs assisted by the passive feedback exhibit excellent robustness and compactness owing to the combination of active amplification and passive feedback from random distributed Rayleigh scattering. However, RFLs with the aid of the passive feedback reported previously are primarily operating in the near-infrared wavelength region of 1.1 μm or 1.5 μm based on the ytterbium or erbium doped active fibers. Here, we propose and demonstrate a random fiber laser configuration combining the passive feedback from a piece of single mode fiber with the active gain based on the thulium doped fiber, and extend the operation wavelength to the critically important mid-infrared wavelength band of ∼2 μm. The spectral and temporal characteristics of the lasing output are evaluated and compared with those generated in a single pass amplification structure and a conventional resonant structure. Our results show that the passive feedback assisted RFL has a stabilized lasing wavelength and notably suppressed temporal dynamics. This work not only advances the development of RFLs, but also provides a promising strategy to achieve mid-infrared laser emission with a comparatively low relative intensity noise. [ABSTRACT FROM AUTHOR]

We report the design and performance of a single frequency all-fiber distributed feedback (DFB) laser sources employing fiber Bragg gratings (FBGs) emitting in the 2- $\mu$ m region. Output powers up to 65 mW CW and optical signal-to-noise ratios of $>$ 65 dB/0.05 nm are obtained. The emission wavelength of the DFB–FBG laser can be fine-tuned with picometer accuracy over a range of 0.6 nm by controlling the pump power and laser temperature. The single frequency source is amplified with polarization-maintaining Ho- or Tm-doped fiber amplifiers to an output power of 1 W CW at 2051 nm or 0.7 W CW at 2039 nm. Heterodyne measurements yield instantaneous laser linewidths of 5 kHz FWHM for both native and amplified signals. [ABSTRACT FROM AUTHOR]

Spectral efficiency (SE) is one of the key metrics for optical communication networks. An important building block for its maximization are optical superchannels, channels that are composed of several subchannels with an aggregate bandwidth larger than the bandwidth of the detector electronics. Superchannels which are routed through the network as a single entity, together with flex-grid routing, allow to more efficiently utilize available bandwidth and eliminate the guard-bands between channels, thus increasing spectral efficiency. In contrast to traditional wavelength division multiplexing (WDM) channels, subchannel spacing and thus superchannel SE is governed by the linewidth and stability of the frequency spacing of the transmitter lasers. Integrated optical frequency combs, particulary the parametrically generated so-called microcombs, which provide optical lines on a fixed frequency grid are a promising solution for low power superchannel laser sources that allow to minimize the SE loss from suboptimal channel spacing. However, it is extremely challenging to realize microcombs with sufficient line power, coherence and line spacing that is compatible with electronic bandwidths. Because the line-spacing generated by most devices is above 40 GHz, demonstrations often rely on additional electro-optic frequency shifter or divider stages to avoid digital-to-analog-converter (DAC) performance degradation when operating at high symbol rates. Here we demonstrate a 52-line superchannel from a single 22 GHz line spacing soliton microcomb. We demonstrate 12 Tb/s throughput with > 10 bits/s/Hz SE efficiency after 80 km transmission and 8 Tb/s throughput (SE > 6 bits/s/Hz) after 2100 km, proving the feasibility and benefits of generating high signal quality, broadband waveforms directly from the output of a micro-scale device with a symbol rate close to the comb repetition rate. [ABSTRACT FROM AUTHOR]

Stimulated Raman scattering (SRS) enabled by the emerging gas-filled low-loss anti-resonant hollow-core fiber (ARHCF) technology opens up a competitive way towards the development of novel lasers in the molecular fingerprint region. In this article, the characteristics of noise and long-term stability of near- and mid-infrared (near-IR and mid-IR) gas-filled fiber Raman lasers have been investigated for the first time. The results reveal that an increase in Raman pulse energy is associated with a decrease in noise, and that the relative pulse peak intensity noise (RIN) is always lower than the relative pulse energy noise (REN). We also demonstrate that long-term drift of the pulse energy and peak power are directly linked with the high amount of heat release during the Raman Stokes generation. The demonstrated noise and long-term stability performance provide necessary references for potential spectroscopic applications as well as further improvements of the emerging IR gas-filled ARHCF Raman laser technology. [ABSTRACT FROM AUTHOR]

Harmonically mode-locked soliton fiber lasers have been intensively investigated in recent years due to their wide range of applications. Drawback of these lasers is relatively large timing jitter which is significantly higher than the timing jitter in lasers operating at fundamental frequency. We report the stabilizing frequency shift effect in harmonic mode-locking ring soliton fiber laser that is studied theoretically and numerically. It is known that a harmonic mode-locking regime in a fiber laser occurs due to interpulse repulsion, which leads to a uniform distribution of pulses in the cavity. We demonstrate that the frequency shift contributes to an increase in the hardness of interpulse interactions and can led to stabilization of the periodic arrangement of pulses. The experiment carried out confirms the theoretical predictions and the results of numerical simulation. [ABSTRACT FROM AUTHOR]

A broadband continuously tunable SESAM all-fiber laser based on optical parametric generation (OPG) for coherent anti-stokes Raman scattering (CARS) imaging applications is presented in this article. This laser structure is built on all-polarization maintaining fibers yielding high environmental stability. The OPG is based on PM photonic crystal fiber as parametric gain pumped by a wavelength-tunable (1025 to 1055 nm) and repetition rate-tunable (3 MHz to 500 KHz) Yb-doped pulsed fiber laser. The all-fiber structure features free polarization adjustment in slow-axis tunable operation with a wavelength range from 770 to 940 nm for signal radiation and 1225 to 1510 nm for idler radiation. The pump and signal pulses (CARS-Stokes and CARS-pump respectively) are intrinsically overlapped, both spatially and temporarily, due to the four-wave mixing (FWM) generation and emitted from a single fiber end. The CARS-Stokes and CARS-pump pulse duration vary between 25 to 32 ps and 21 to 24 ps, respectively in the whole band, ensuring a good overlap of the generated signals, and both exhibit a Gaussian profile with high spatial beam quality. The frequency conversion allows to obtain a frequency difference between the generated signals from 1100 to 3300 cm−1. The reported low-cost laser source is ideal for bio-imaging applications, especially in CARS imaging, due to its compactness, simplicity, robustness and ease of handling. A patent has been filed based on this technology. [ABSTRACT FROM AUTHOR]

Further power scaling and stable laser performance were demonstrated in the Yb/Ce codoped aluminosilicate fiber fabricated through low-temperature chelate gas phase deposition technique. The molar ratio of Ce/Yb was designed and optimized to be 0.58 for low background loss, effective photodarkening suppression, and no additional thermal load. The background loss of this active fiber was 4.7 dB/km and its photodarkening loss at equilibrium was as low as 3.9 dB/m at 633 nm. Benefiting from low-temperature deposition technique, the fiber showed uniform core composition devoid of clustering and central ‘dip’ of refractive index profile and 0.19 mol% Yb2O3 was homogeneously dissolved into the fiber core plus with 0.41 mol% Al2O3, 0.11 mol% Ce2O3, and 0.32 mol% SiF4. Based on a master oscillator power amplifier laser setup, 5.04 kW laser output at 1079.80 nm was achieved with a slope efficiency of 81.1%. Stabilized at 5kW-level laser for over 60 minutes, the output power presented almost no power degradation, directly confirming a noticeable photodarkening mitigation. [ABSTRACT FROM AUTHOR]

Multi-wavelength distributed feedback laser array based tunable semiconductor lasers have been widely used owing to their high mode stability and simple tuning scheme. Comparing with the in-parallel laser array, the in-series one can avoid a large power loss caused by the optical combiner. However, the single-mode stability of an in-series laser array is vulnerable to the reflections of other lasers. Here we experimentally demonstrated a tunable in-series laser array with high single-mode stability and high uniformity of wavelength spacing. To reduce the impact of such reflections, the Bragg gratings in different lasers were designed in phase, and the grating phase error was reduced by utilizing the reconstruction-equivalent-chirp technique. Besides, a three-section laser structure was applied in each laser to increase the priority of the dominant lasing mode. Four lasers with small wavelength-spacings of 2.5 nm were integrated in the in-series laser array. According to the measurement results of 40 in-series laser arrays (160 lasers), wavelength deviations of 90.6% lasers were within ±0.2 nm, average wavelength spacings of 97.5% of the measured laser arrays were deviated from the design within ±0.2 nm, and SMSRs of 96.3% lasers were above 45 dB. The output power was above 25 mW and the relative intensity noise was below −130 dB/Hz. Like the in-parallel laser array, the proposed laser array can be applied in either continuous wavelength tuning or fast channel switching. For the continuous wavelength tuning applications, the 10-nm tuning range was obtained with a temperature tuning range of less than 20 °C. For the fast channel switching applications, four channels with 2.5-nm spacing are available, and the switching time between two channels was less than 300 ns. [ABSTRACT FROM AUTHOR]

In this paper we focus on analyzing the accuracy of time transfer in bidirectional fiber optic links. It has been pointed out that one of the main uncertainty contributions in such links is related to insufficient stabilization of the lasers’ wavelengths and to the asymmetry of propagation caused by the fiber's chromatic dispersion. To reduce these contributions, we propose a scheme where the difference (offset) of the wavelengths of forward and backward lasers is stabilized and where optical interleavers are used as diplexers. This allows implementing a λ-swapping technique to assess dispersion-related link asymmetry during the link calibration. For the investigation of these approaches and the measurement of the associated stability we utilize a frequency-synchronized offset laser module (FSOLM) to realize an offset of 25 GHz. In this paper, we show that with the proposed λ-swapping technique the uncertainty associated with the propagation asymmetry due to fiber chromatic dispersion can be reduced to values below 2.5 ps for links up to 1000 km long, making it insignificant compared to other uncertainty contributions. A laboratory proof-of-concept experiment, in which distances up to 540 km were tested, showed good agreement between the measured and the anticipated propagation delay of investigated links. [ABSTRACT FROM AUTHOR]

We report on the free-running and frequency stabilized operation of a ring resonator-tuned full-band tuneable laser. The laser is a monolithically integrated semiconductor ring laser tuneable over 34 nm, from 1522–1556 nm, fabricated using a commercially-available, InP-based, active-passive foundry technology. The mode selection is implemented using the Vernier effect by reverse-biasing voltage-controlled electro-optically tuned ring resonators. The laser exhibits a typical intrinsic linewidth of 110 kHz over the C-band and a side-mode suppression ratio (SMSR) exceeding 50 dB. The linewidth over the tuning range does not significantly vary, showing that the tuning mechanism does not cause linewidth broadening. The power dissipated in the tuning elements is 1.4 mW per ring. Using the same reverse bias voltage-controlled electro-optic tuning, we also demonstrate the locking of the laser to a high-finesse etalon using Poun–Drever–Hall frequency locking. Our implementation requires a single control loop because of the reverse bias tuning. All deployed tuning effects in the phase modulators have the same sign therefore resulting in a flatter frequency dependent phase response compared to current injection feedback. The control loop has a bandwidth of 500 kHz and a control range of 2.9 GHz. We report 1 kHz level linewidth for millisecond observation times for the stabilized laser. [ABSTRACT FROM AUTHOR]

The spectrum of Optical Frequency Combs (OFC) consists of a series of closely spaced optical frequencies that spread over tens to hundreds of nanometers. Stimulated Brillouin Scattering (SBS) can be used to filter a single-mode from the OFC spectrum with superior amplification; however, the stability of the filtered mode can be degraded to the level of 10−11 in the course of the SBS filtration process. This prevents the employment of this technique in high precision experiments. In this paper, a phase locking scheme is introduced to enhance the relative stability and accuracy of the filtered mode with respect to its original one. After application of this scheme, the relative stability between both modes reaches 10−17 in 100 seconds; while, the relative accuracy is measured to be 3.6 × 10−19. [ABSTRACT FROM AUTHOR]

Transition metal dichalcogenides have recently been considered the candidates for saturable absorption materials due to their advantages in both electronic and optoelectronic. Compared with MoS2 and WS2, MoTe2 exhibits smaller bandgap. As a result, MoTe2 owns inherent advantages in the near-infrared applications. In this paper, the MoTe2 saturable absorber (SA) were prepared by the magnetron sputtering deposition method. Owing to the enhanced light-materials interaction between MoTe2 and evanescent field from the microfiber, the modulation depth of the MoTe2 SA was up to 26.97%. By assembling the MoTe2 SA into the erbium-doped fiber laser, the mode-locked fiber laser at 1.5 μm was demonstrated. The pulse duration of 111.9 fs was proved to be the shortest in the fiber lasers based on transition metal dichalcogenides. Moreover, the mode-locked fiber laser maintains the long-term stability. Our results suggest that the proposed MoTe2 SA is promising for the ultrashort pulse generation and stable system operation. [ABSTRACT FROM AUTHOR]

Semiconductor lasers that use high-order surface gratings formed with etched slots have simple fabrication. Such gratings however increase loss and have a small free-spectral range that can increase threshold, lower slope efficiency, and limit SMSR. In this paper, we look at the design of a grating and use 2, 3, and 4 period structures within a 37 ${\text{th}}$ order grating. At fixed temperature, a single period device should work best while temperature tuning requires a multiperiod device. We found that the two period grating structure works best among the multiperiod devices. A follow-on study with the scattering matrix model shows that the slope efficiency which is quite low in this study (about 0.1 mW/mA at best) can be improved to a value of upto 0.17 mW/mA by reducing the number of slots used to form the grating from the present design that has 24 slots to 18. [ABSTRACT FROM AUTHOR]

A single frequency ring fiber laser is reported based on a cladding-pumped ytterbium-doped fiber (YDF). A medium concentration polarization maintaining (PM) YDF is utilized in ring cavity as a saturable absorber that acts as an ultranarrow bandwidth filter. Here, by making use of double-clad and PM fibers in the cavity configuration, thermal noises are notably damped and the frequency instability is minimized due to the temperature fluctuation and polarization variation. The laser yields up to 35 mW of a stable single longitudinal mode output at 1079.68 nm, whereas the corresponding linewidth is estimated less than 6 kHz. Subsequently, a fiber amplifier is employed to boost up power to 5.21 W with the slope efficiency of 45.6%. The amplifier steadily operates below the stimulated Brillouin scattering threshold power. The signal-to-noise ratio is measured to be higher than 70 dB, while the laser linewidth remains unchanged. The relative intensity noise of the laser at the relaxation oscillation frequency of 173 kHz is determined to be −115 dB/Hz, which approaches to −134 dB/Hz at frequencies over 500 kHz. [ABSTRACT FROM AUTHOR]

This paper describes an optical phase lock loop (OPLL) implemented as an ultraselective optical frequency comb line filter. The OPLL is based on a photonic integrated circuit (PIC) fabricated for the first time through a generic foundry approach. The PIC contains a distributed Bragg reflector (DBR) laser whose frequency and phase are stabilized by reference to an optical frequency comb generator. The OPLL output is a single-mode DBR laser line; other comb lines and noise at the output of the OPLL filter are attenuated by >58 dB below the peak power of the OPLL-filter output line. The OPLL bandwidth is up to 200 MHz, giving a filter quality factor greater than 1,000,000. The DBR laser can be tuned over 1 THz (8 nm), enabling different comb lines to be selected. Locking to a comb line with a frequency offset precisely selectable between 4 and 12 GHz is also possible. The coherence between the DBR laser and the comb lines is demonstrated by measurements of the heterodyne signal residual phase noise level, which is below −100 dBc/Hz at 5 kHz offset from the carrier. The OPLL-filter output can be up to 6 dB higher than the peak power of the comb line to be isolated by the filter. This optical gain is a unique characteristic which can significantly improve the SNR of communication or spectroscopy systems. This OPLL is envisaged to be used for high purity, tuneable microwave, millimetre-wave, and THz generation. [ABSTRACT FROM AUTHOR]

Random numbers are an essential resource to many applications, including cryptography and Monte Carlo simulations. Quantum random number generators (QRNGs) represent the ultimate source of randomness, as the numbers are obtained by sampling a physical quantum process that is intrinsically probabilistic. However, they are yet to be widely employed to replace deterministic pseudo random number generators (PRNG) for practical applications. QRNGs are regarded as interesting devices. However, they are slower than PRNGs for simulations and are typically seen as clumsy laboratory prototypes, prone to failures, and unreliable for cryptographic applications. Here, we overcome these limitations and demonstrate a compact and self-contained QRNG capable of generating random numbers at a pace of 8 Gbit/s uninterruptedly for 71 days. During this period, the physical parameters of the quantum process were monitored in real time by self-checking functions implemented in the generator itself. At the same time, the output random numbers were analyzed with the most stringent suites of statistical tests. The analysis shows that the QRNG under test sustained the continuous operation without physical instabilities or hardware failures. At the same time, the output random numbers were analyzed with the most stringent suites of statistical tests, which were passed during the whole operation time. This extensive trial demonstrates the reliability of a robustly designed QRNG and paves the way to its use in practical applications based on randomness. [ABSTRACT FROM AUTHOR]

We demonstrate a technique to precisely control and stabilize the beat frequency of a photonic microwave signal generator based on beating the optical signals of two lasers on a high-speed photodetector. The approach does not require high-speed electronic circuitry, but allows the control of the generated signal frequency up to hundreds of GHz. The demonstrated technique can readily be integrated on a chip-scale device using heterogeneous silicon platform, opening possibilities for widespread use in signal generation, sensing, instrumentation, and communications. [ABSTRACT FROM PUBLISHER]

The goal of this study is to establish a phase comparison method using high-frequency microwave photonic signals. The signal is generated as a beat frequency of a dual-wavelength optical signal. We propose a double differential phase comparison method suitable for a phase-locked loop. This is achieved by using an optical frequency shifter and two low-frequency photomixers (optical-to-electrical signal converters). The basic approach of the method is that the phase difference between the high-frequency dual-wavelength signals can be measured as a differential phase of the detected low-frequency microwave signals by two respective low-frequency photomixers without high-frequency photomixers. With this approach, the phase comparison of any range of frequencies of the dual-wavelength signals can be performed by two identical low-frequency photomixers. In an application of this method for separate fiber-coupled stations, a common laser signal transmission is effectively carried out with a dual-wavelength optical signal. We conducted an experiment to verify the effectiveness of the double differential phase comparison method by comparing the direct phase comparison method. [ABSTRACT FROM PUBLISHER]

In this paper, we present a method of self-detection of mode hops in a distributed Bragg reflector tunable laser using the voltage drop in the active section as a monitoring signal. Based on this method, a wavelength setting and stabilization algorithm that is tolerant to temperature variations is proposed. Wavelength locking for a period of more than seven days with an accuracy better than ±1.25 GHz has been achieved, meeting the ITU-T standard for 25 GHz channel spacing. Under forced changes in the operation temperature from 15° to 60°, with variations of 0.1°/s, the maximum allowed deviation of ±2.5 GHz for the ITU-T standard with 50 GHz channel spacing was kept. [ABSTRACT FROM PUBLISHER]

We demonstrate the short- and long-term stable operation of an all-polarization-maintained Fabry–Pérot cavity passively mode-locked fiber laser. The laser operates in an all-anomalous-dispersion solitonic regime. Laser stability is studied by a variety of measurements, which confirm the high stability of the laser in the temporal and spectral–both optical and electrical-domains. Pulse durations of 540 fs, period-relative time jitters of $\sim$0.015‰, and long-term uninterrumped operation with 0.4% variation (standard deviation) in the average output power are obtained. The highly stable operation of the laser oscillator was maintained after amplifying the laser output with a conventional EDFA. Pulse durations of $\sim$244 fs, period-relative time jitters of $\sim$0.019‰,  and an average output power of 20 mW were obtained after amplification, while maintaining the 100-dB signal-to-noise ratio of the laser oscillator measured at 500-Hz offset from the fundamental harmonic frequency. The theoretical validation of our experimental results is based on solutions of the Nonlinear Schrödinger Equation. We demonstrate that wavelength and z -position dependences of the active medium gain must be taken into account for an accurate correspondence with the experimental properties of the laser. [ABSTRACT FROM PUBLISHER]

A single-longitudinal-mode (SLM) fiber ring laser based on topological insulator (TI) Bi2Te3 saturable absorber (SA) had been experimentally demonstrated. The TI: Bi2Te3 nanosheets, which were fabricated through the bottom-up approach, were coated onto the microfiber as an efficient saturable absorber device to ensure the SLM operation. Combined with fiber Bragg grating, the output wavelength can be continuously tuned about 1 nm. Without any accurate cavity control, stable SLM operation without mode hopping had been conveniently achieved. Based on the Lorentzian fitting, the line-width was measured to be less than 10 kHz. The output power of SLM fiber laser can reach up to 23 mW, indicating that the proposed TI-based SA may be suitable for high-optical power SLM operation in the future. [ABSTRACT FROM PUBLISHER]

We report a novel method to generate a stable and frequency-hopping-free microwave signal based on a mutually injection-locked dual-wavelength single-longitudinal-mode fiber laser and an optoelectronic oscillator (OEO), with the mutual injection locking realized by sharing an optical path consisting of a polarization modulator and a polarization-maintaining phase-shifted fiber Bragg grating. The two wavelengths from the fiber laser are injected into the OEO to lock the generated microwave signal, while the microwave signal from the OEO is fed back into the fiber laser to injection lock the two wavelengths. Thanks to the mutual injection locking, the operation stability of the fiber laser and the OEO are substantially improved. A microwave signal at 11.8 GHz with a phase noise of -105 dBc/Hz at a 10-kHz offset frequency is generated. A stable operation of the system without frequency shifting and hopping is demonstrated. [ABSTRACT FROM PUBLISHER]

An innovative external cavity dual-mode laser has been developed for continuous-wave terahertz generation via photomixing. Photomixing is a method of generating terahertz radiation at the beat frequency developed by two input optical light modes. Dual-mode lasers are an intriguing option for photomixing sources because the differential frequency produced is stabilized by the common mode rejection ratio effect. The laser presented here utilizes an acousto-optic tunable filter for frequency selection, which creates an ultra-wideband tunable range and provides the potential for tuning speed on the order of microseconds. The unique design of this laser enables 100% solid state operation resulting in a robust and compact system for applications in terahertz imaging or spectroscopy. Operating at around 1550 nm, this laser is capable of producing differential frequencies between 300 GHz and 18 THz with a side-mode-suppression-ratio of 50 dB and peak amplitudes of over 0 dBm. The device presented here represents a significant achievement in the advancement of dual mode lasing technology for tunable terahertz generation with high phase and amplitude stability. [ABSTRACT FROM PUBLISHER]

An erbium fiber oscillator with dominating gain fiber length and an intracavity pulse shaper is experimentally demonstrated. The Kerr nonlinear phase shift induced in the fibers is suppressed by the pulse shaper, enabling high-energy (8.9 nJ) high-compression efficiency (93%) pulses of less than 270 fs duration (after linear dechirping) at modest (320 mW) pump power. A stably mode-locked spectrum corresponding to 19 nJ, 83 fs transform-limited pulse is achieved at 720 mW pump power. Wave-breaking-free pulse energy can be further enhanced by introducing stronger spectral phase modulation via the pulse shaper if more pump budget is available. We also demonstrate manipulation of the central wavelength and bandwidth of the output pulse by intracavity spectral amplitude modulation via the pulse shaper. [ABSTRACT FROM PUBLISHER]

We measured the bit error rate (BER) of all-optical flip-flop operations using a 1.55-μm polarization bistable vertical-cavity surface-emitting laser (VCSEL). Polarization bistable flip-flop operations were obtained by injecting optical set and reset pulses with independently adjusted wavelengths toward the two VCSEL lasing modes, whose polarizations are orthogonal to each other. Acquired waveforms showed clearly opened eyes, and BERs less than 1 × 10-9 were measured up to 1 Gb/s although the optical set and reset pulse power was much lower than the VCSEL output power. We also demonstrated all-optical flip-flop operations by injecting optical set and reset pulses with the same wavelength, which were generated by a single laser diode. In this case, we obtained polarization bistable flip-flop operations up to 500 Mb/s with almost the same BER values that were obtained when the wavelengths of the set and reset pulses were adjusted independently. The optical input pulse powers were higher than for the case using input pulses with two different wavelengths, but still lower than the VCSEL output power. [ABSTRACT FROM PUBLISHER]

A novel configuration for a dual-wavelength fiber ring laser with improved lasing stability realized through the use of an injection-coupled optoelectronic oscillator (OEO) is proposed and demonstrated, and its application to transverse load sensing is studied. The OEO-coupled dual-wavelength laser has two mutually coupled loops: the fiber ring loop and the OEO loop. In the fiber ring loop, a polarization-maintaining phase-shifted fiber Bragg grating is incorporated to generate two optical wavelengths with the wavelength spacing determined by the birefringence of the polarization-maintaining (PM) fiber. In the OEO loop, a microwave signal with its frequency also determined by the birefringence of the PM fiber is generated, which is fed into the fiber ring loop to injection lock the dual wavelengths. Due to the injection locking, a very stable dual-wavelength operation is established. The use of the dual wavelengths for high-resolution and high-speed transverse load sensing is then implemented. The sensitivity of the transverse load sensor is measured as high as +9.7573 and -9.7350 GHz/(N/mm), along the fast and slow axes, respectively. The high frequency purity and stability of the generated microwave signal permits very reliable and high accuracy measurement and the microwave frequency interrogation allows the system to operate at an ultra-high speed. [ABSTRACT FROM PUBLISHER]

A novel double-pumped ring cavity multiwavelength fiber optical parametric oscillator (MW-FOPO) with tunable wavelength spacing is proposed. In the MW-FOPO, we utilize a highly nonlinear dispersion-shifted fiber as the gain medium and a Mach-Zehnder interferometer as the comb-like filter. Twenty-four-wavelength lasing of the double-pumped MW-FOPO with a ripple less than ±4.3 dB and a wavelength spacing of about 0.8 nm in a wavelength range from 1541 to 1558 nm is experimentally demonstrated. The wavelength spacing can be continuously tuned and multiwavelength lasings with wavelength spacings of 0.08, 0.2, 0.4, and 0.8 nm are demonstrated, respectively. We discussed the power stability of the multiwavelength lasing of the double-pumped MW-FOPO. A comparison of the output spectra among the double-pumped MW-FOPO, the single-pumped MW-FOPO, and the multiwavelength erbium-doped fiber laser is also presented. [ABSTRACT FROM PUBLISHER]

We experimentally confirm that graphene within fiber laser cavities can generate four-wave-mixing (FWM) by observing the laser spectral broadening and the transition from the single-longitudinal-mode oscillation to multiple-longitudinal-mode one. Then, by simultaneously exploiting the graphene-induced nonlinear FWM and its super-broadband saturable absorption, we further achieve for the first time to the best of our knowledge, multiwavelength Q-switched Yb^3+- or Er^3+-doped fiber lasers at 1 \mum and 1.5 \mum wavebands, respectively. Simultaneous 23-wavelength Q-switching oscillation with a wavelength spacing of 0.2 nm is stably generated at 1.5 \mum waveband. The multiwavelength Q-switched pulses have the minimum pulse duration of 2.5 \mus, the maximum pulse energy of 72.5 nJ and a wide range of pulse-repetition-rate (PRR) from 2.8 to 63.0 kHz. At 1 \mum waveband, we also obtain five-wavelength simultaneous lasing in Q-switching regime with the pulse duration of \sim3\ \mus, pulse energy of 10.3 nJ and PRR between 39.8 and 56.2 kHz. [ABSTRACT FROM PUBLISHER]

We present a power stabilization method for a dual-mode Ti:Sapphire ring cavity laser. Without stabilization, mode competition caused the output power of the two modes to fluctuate by approximately 10 dB. The power stabilization system, which used volume holographic gratings to monitor the power in each mode, reduced the power fluctuations to less than 3 dB. [ABSTRACT FROM PUBLISHER]

We demonstrate an all-optical wavelength-division-multiplexing ring network immune to transients based on individual optically gain clamped waveguide amplifiers. The ring network is compared to a similar point-to-point configuration to quantify the effect of recirculating amplifier noise. In the results, we notice that the optical stabilization is strong enough to overcome the effect of accumulated amplifier noise over the entire transmission bandwidth, and no lasing or chaotic behavior occurs. In addition, due to the independent amplifier stabilization, the network has shown to be robust even in the case of failure and restoration. We prove that the obtained results are scalable with the amplifier number. All results are also applicable to fiber-based amplifiers. [ABSTRACT FROM PUBLISHER]

This paper presents rapid, precise, and stable control of optical frequency for sampled-grating distributed Bragg reflector (SG-DBR) laser diode (LD) using a Z-cut quartz etalon supported at the middle point. The optical frequency of SG-DBR LD was controlled by the mode-hop-free three-electrode control method. As a result, stable mode operation and wide continuous tuning range without mode hopping have been realized. A 0.2-ms optical frequency tuning speed with a 4.35-THz tuning range and a 130-MHz control error of the optical frequency was demonstrated. A 1.1-MHz Allan deviation was reached at an integration time of 12 times 103 s. [ABSTRACT FROM PUBLISHER]

We examine the dynamics of semiconductor lasers coupled in a ring configuration. The lasers, which have stable output intensity when isolated, behave chaotically when coupled unidirectionally in a closed chain. In this way, we show that neither feedback nor bidirectional coupling is necessary to induce chaotic dynamics at the laser output. We study the synchronization phenomena arising in this particular coupling architecture and discuss its possible application to chaos-based communications. Next, we extend the study to bidirectional coupling and propose an appropriate technique to optical chaos encryption/decryption in closed chains of mutually coupled semiconductor lasers. [ABSTRACT FROM PUBLISHER]

A stabilized and tunable single-longitudinal-mode erbium-doped fiber ring laser has been proposed and experimentally demonstrated. The laser is structured by combining the compound cavity with a fiber Fabry-Peacuterot tunable filter. An injection-locking technique has been used to stabilize the wavelength and output power of the laser. One of the longitudinal modes is stimulated by the injected continuous wave so that this mode is able to win the competition to stabilize the system. A minimum output power of 0.6 dBm and a signal-to-noise ratio of over 43 dB within the tuning range of 1527-1562 nm can be achieved with the proposed technique. A wavelength variation of less than 0.01 nm, a power fluctuation of less than 0.02 dB, and a short-term linewidth of about 1.4 kHz have also been obtained [ABSTRACT FROM PUBLISHER]

With the aim of developing a fast wavelength-tunable laser for advanced dynamic network reconfigurations or optical burst switching, a short-cavity (SC) distributed-Bragg-reflector (DBR) laser array with an active-DBR (ADBR) structure (SC-ADBR laser) was fabricated. First, it was experimentally confirmed that the ADBR structure compensates for optical loss and achieves stable single-mode operation during wavelength tuning. An artificial-laser-phase-adjustment technique in the SC-ADBR laser by precise adjustment of a DBR mirror was then proposed. By applying this technique to a six-channel SC-ADBR laser array, perfect wavelength accessibility across a 21-nm range was attained [ABSTRACT FROM PUBLISHER]

The relative intensity noise of vertical-cavity surface-emitting lasers (VCSELs) subject to polarization-resolved optical feedback has been investigated experimentally. A comparison is made of the effects of polarization-selective and polarization-preserving optical feedback on the polarization stability of VCSELs. The experiments show that polarization-selective optical feedback has a greater effect on the dynamics of VCSELs than polarization-preserved optical feedback. Polarization-selective optical feedback can be used to control the polarization stability of the VCSEL operating near the polarization-switching current and is easier to affect the dynamics of the VCSEL in the polarization stable regime [ABSTRACT FROM PUBLISHER]

This paper describes the design and operation of a stable narrow-linewidth linearly polarized fiber ring laser using a polarization-maintaining (PM) erbium-doped fiber as a saturable absorber. The effect of the PM fiber on suppressing mode hopping is experimentally demonstrated and optimum conditions for single-mode operation are identified. Laser output power is ∼ 4.7 mW at 1535 nm for a pump power of 94 mW, the polarization extinction ratio is 24.8 dB, the SNR is larger than 45 dB, the relative intensity noise is below -104 dB/Hz at frequencies above 150 kHz, and the linewidth is less than 1.5 kHz. Potential applications of the fiber laser for interferometric or spectroscopic fiber sensors are briefly discussed. [ABSTRACT FROM PUBLISHER]

The optical characteristics of antiresonant reflecting optical waveguide (ARROW) vertical-cavity semiconductor optical amplifiers (VCSOAs) are investigated theoretically. Influence of spatial hole burning (SHB) and thermal lensing on the stability of the amplified output signal is taken into consideration in the studies. A design rule, which can be used to deduce the optimized dimensions of ARROW, is developed to achieve spatially stable single-spot amplified output-signal in large-area VCSOAs. It can be shown that a high saturation power yet stable output signal can be achieved in large-area ARROW VCSOAs simultaneously. [ABSTRACT FROM PUBLISHER]

Theoretical investigations on commanding the bistability and self-pulsation characteristics of two-segment bistable semiconductor laser diode (TBLD) using delayed optoelectronic feedback have been carried out in this paper. The effects of feedback gain and delayed time have been simulated numerically based on the rate equations. The results show that the stable region of TBLD varies periodically with delayed time, the bistable region shrinks with the increase of delayed time, stability can be enhanced for certain values of delayed time, and instability can be reached easily for negative delayed optoelectronic feedback. These conclusions establish the theoretical foundation to optimize and control the TBLD and may have potential applications in bistable switching and self-pulsation generation. [ABSTRACT FROM PUBLISHER]

The dynamics of a semiconductor laser subject to polarization-rotated (incoherent) optical feedback in the long cavity limit has been numerically and experimentally investigated. The results show that the induced dynamics can be grouped into four regimes (stable, chaotic, pulsed, and two state) and that the transverse-magnetic mode of the laser never lases in the ring cavity configuration studied. In addition, unlike the earlier short-cavity work, the boundaries between the regimes in the long-cavity case were found to be independent of the external-cavity delay time. [ABSTRACT FROM PUBLISHER]