Mode Hopping Research Articles

Sub-kHz-linewidth continuous-wave single-frequency ring-cavity fiber laser based on high-gain Er: YAG crystal-derived silica fiber.

Throughout the development of single frequency fiber lasers (SFFLs), gain fiber is one of the most important components, which can greatly affect the quality of SFFLs. Here, we fabricated an Er: YAG crystal-derived silica fiber (EYDSF) using a CO2 laser-heating drawing technique, with a high gain coefficient of 1.74 dB/cm. Employing the EYDSF of only 10 cm as a gain medium, we constructed a continuous-wave ring-cavity SFFL with an all-fiber system. An ultra-narrow linewidth <660 Hz was achieved harnessing a homemade low-concentration Er-doped silica fiber as a saturable absorber. Importantly, the SFFL output power was up to 32.7 mW at 1560 nm. What's more, no multi longitudinal mode or mode hopping were found in 2 hours, and the fluctuation of power was <0.63% in 8 hours. Furthermore, the relative intensity noise was lower to -145 dB/Hz at frequencies over 1 MHz. The results indicate that the ring-cavity SFFL has desirable performance in output power, linewidth, stability and noise, which serves a prospective candidate applied to coherent optical communications, high-precision sensors, laser radars and other advanced fields.

Laser system of cold atom optical clock in China Space Station

The world's first space optical clock (SOC) developed in China, which is composed of five subsystems, i.e. an optical unit, a physics unit, an electronic control unit, a space optical frequency comb, and an ultrastable laser, was successfully launched with the Mengtian space laboratory on October 31, 2022, and entered into the China Space Station (CSS). Compact and stable laser is a key element for the operation of the SOC. The optical unit consists of 5 lasers with wavelengths of 461, 679, 689, 707 and 813 nm, respectively. With a synchronous-tuning-like scheme, high-quality external cavity diode lasers (ECDLs) are developed as the seeds. The linewidths of the lasers are all reduced to approximately 100 kHz, and their tuning ranges, free from mode hopping, are capable of reaching 20 GHz, satisfying the requirements for the SOC. With careful mechanical and thermal design, the stability of the laser against vibration and temperature fluctuation is sufficiently promoted to confront the challenge of rocket launching. While the power from the ECDL is sufficient for 679-nm repump laser and 707-nm repump laser, additional injection lock is utilized for the 461-nm laser and 689-nm laser to amplify the power of the seeds to more than 600 mW, so that effective first and second stage Doppler cooling can be achieved. To generate an optical lattice with deep enough potential well, over 800-mW 813-nm lasers are required. Therefore, a semiconductor tapered amplifier is adopted to amplify the seed to more than 2 W, so as to cope with various losses of the coupling optics. The wavelengths and output power values of the 5 lasers are monitored and feedback is controlled by the electronic control unit. All the modules are designed and prepared as orbital replaceable units, which can be easily replaced by astronauts in case failure occurs. Now the lasers are all turned on and operate normally in CSS. More data of the SOC will be obtained in the near future. At present stage, according to our evaluation, the continuous operation time of the SOC is limited by the injection locked lasers, which are relatively vulnerable to mode hopping. Hopefully, this problem can be solved by improving the laser diode preparing technology, or developing fiber lasers with compact frequency conversion modules.

A Passively Wavelength-Stabilized Mid-Infrared Optical Parametric Oscillator

High-power and widely tunable continuous-wave optical parametric oscillators (cw OPOs), as mature radiation sources in the mid-infrared range, are limited by their frequency fluctuation. We built a cw OPO working at the near- and mid-infrared ranges and discuss the extent to which its frequency stability is affected by the mechanical temperature and the air pressure of the cavity. In addition, we artificially provoked mode hops by changing the above two factors to analyze the thermal-induced refractive index variation of the crystal. An inequality between the spacing of the hops and the free spectrum range (FSR) was observed. The wavelength stability of the cw OPO was guaranteed passively, and the longest mode-hopping-free time exceeded 13 h through sealing the cavity and thermal control. Further analysis of factors affecting frequency drift is critically essential for the reliability of cw OPO, particularly in environments outside the laboratory, and may provide a new method for high-precision tuning wavelengths.

Compact, Watt-class 785 nm dual-wavelength master oscillator power amplifiers

785 nm micro-integrated, dual-wavelength master oscillator power amplifiers with a footprint of 5 mm × 25 mm are presented. They are based on Y-branch distributed Bragg reflector ridge waveguide diode lasers and anti-reflection coated tapered amplifiers. In order to reduce the impact of potential optical feedback, devices with master oscillator front facet reflectivities of 5% and 30% as well as with an integrated miniaturized optical isolator have been realized. A comparison up to 1 W shows narrowband dual wavelength laser emission with a spectral distance of 0.6 nm (10 cm−1) and individual spectral widths <20 pm. As expected, a higher front facet reflectivity leads to a significant reduction of feedback related mode hops. Longitudinal modes corresponding to the master oscillator resonator length remain within spectral windows <0.15 nm (3 cm−1), suitable for applications such as Raman spectroscopy and especially shifted excitation Raman difference spectroscopy. Integrating a compact 30 dB optical isolator completely eliminates the observed optical feedback effects. Lateral beam propagation ratios of 1.2 (1/e2) enable easy beam shaping and fiber coupling. Outside of the experimental comparison, the developed MOPAs provide up to 2.7 W of optical output power available for applications.

A narrow-linewidth three-mirror VCSEL for atomic devices

We consider the use of a recently invented three-mirror vertical-cavity surface-emitting laser (VCSEL) as an optical-pumping light source for next-generation atomic devices (e.g., atomic clocks). While VCSELs are attractive given their very low size, weight, and power and intrinsic resistance to longitudinal mode hops, their relatively large linewidths give rise to efficient laser phase-noise to transmitted intensity noise conversion (PM-to-AM), which severely limits the signal-to-noise ratio in vapor-cell signals. In contrast, the three-mirror VCSEL has a linewidth more than ten times narrower than traditional VCSELs. Using a three-mirror VCSEL in a continuous-wave Cs vapor-cell atomic clock testbed, we demonstrate the utility of this laser for next-generation atomic devices in general, obtaining a short-term stability of 3.6 × 10−12/τ1/2 without any attention to PM-to-AM mitigation strategies.

Parity-time-symmetric optoelectronic oscillator based on higher-order optical modulation.

An optoelectronic oscillator (OEO) for single-frequency microwave generation, enabled by broken parity time (PT) symmetry based on higher-order modulation using a Mach-Zehnder modulator, is proposed and demonstrated. Instead of using two physically separated mutually coupled loops with balanced gain and loss, the PT symmetry is realized using a single physical loop to implement two equivalent loops with the gain loop formed by the beating between the optical carrier and the ±1st-order sidebands and the loss loop formed by the beating between the ±1st-order sidebands and the ±2nd-order sidebands at a photodetector. The gain and loss coefficients are made identical in magnitude by controlling the incident light power to the modulator and the modulator bias voltage. Once the gain/loss coefficient is greater than the coupling coefficient, the PT symmetry is broken, and a single-frequency oscillation without using an ultra-narrow passband filter is achieved. The approach is evaluated experimentally. For an OEO with a loop length of 10.1 km, a single-frequency microwave signal at 9.997 GHz with a 55-dB sidemode suppression ratio and -142-dBc/Hz phase noise at a 10-kHz offset frequency is generated. No mode hopping is observed during a 5-hour measurement period.

GaSb surface grating distributed feedback interband cascade laser emitting at 3.25 µm.

A second-order distributed feedback interband cascade laser emitting at 3.25 µm was designed, grown, and fabricated. By coherent epitaxy of a GaSb cap layer instead of the conventional thin InAs cap on top of the laser structure, a high-quality surface grating was made of GaSb and gold. Enough coupling strength and a significant inter-modal loss difference were predicted according to the simulation within the framework of couple-wave theory. Lasers having 2-mm-long cavities and 4.5-µm-wide ridges with high-/anti-reflection coatings were fabricated. The continuous-wave threshold current and maximum single-mode output power were 60 mA and 24 mW at 20°C, respectively. The output power of 5 mW was still kept at 55°C. Continuous tuning free from mode hopping and high single-mode suppression ratios (>20 dB) were realized at all injection currents and heat-sink temperatures, covering a spectral range of over 20 cm-1.

Spectral splicing for an OFDR sensing system using a DBR laser.

A spectral splicing method is proposed for an optical frequency domain reflectometry (OFDR) sensing system using a distributed Bragg reflector (DBR) laser. An external cavity tunable laser (ECTL) is used in a conventional OFDR sensing system. There are no reports to our knowledge about using a DBR laser as the light source for an OFDR sensing system. A DBR laser has the advantages of small size, no mechanical scanning, easy integration, and low cost. However, due to the mode hopping, the wavelength can only be continuously tuned in the range of about 1 nm. The spectral splicing method was used to splice 39 partially overlapping Rayleigh scattering signals into a continuous signal with wavelength of 35.174 nm. Distributed optical fiber strain sensing with 5 mm spatial resolution is realized. The strain range is ±2500µε, and the maximum error of the measured strain is 9.91 µε.

Performance optimisation of a holographic Fourier domain diffuse correlation spectroscopy instrument.

We have previously demonstrated a novel interferometric multispeckle Fourier domain diffuse correlation spectroscopy system that makes use of holographic camera-based detection, and which is capable of making in vivo pulsatile flow measurements. In this work, we report on a systematic characterisation of the signal-to-noise ratio performance of our system. This includes demonstration and elimination of laser mode hopping, and correction for the instrument’s modulation transfer function to ensure faithful reconstruction of measured intensity profiles. We also demonstrate a singular value decomposition approach to ensure that spatiotemporally correlated experimental noise sources do not limit optimal signal-to-noise ratio performance. Finally, we present a novel multispeckle denoising algorithm that allows our instrument to achieve a signal-to-noise ratio gain that is equal to the square root of the number of detected speckles, whilst detecting up to ∼1290 speckles in parallel. The signal-to-noise ratio gain of 36 that we report is a significant step toward mitigating the trade-off that exists between signal-to-noise ratio and imaging depth in diffuse correlation spectroscopy.

Stabilized Narrow-Linewidth Brillouin Random Fiber Laser With a Double-Coupler Fiber Ring Resonator

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.

Continuous-wave operation of 405 nm distributed Bragg reflector laser diodes based on GaN using 10th-order surface gratings

Single longitudinal mode continuous-wave operation of GaN-based distributed Bragg reflector (DBR) laser diodes with 10th-order surface gratings is demonstrated. The DBR consists of periodic V-shaped grooves on a 2 μm wide ridge waveguide fabricated by using electron-beam lithography and plasma etching. The effect of different lengths of the DBR section and the gain section on the device performance has been studied. Periodic mode hops to the adjacent longitudinal Fabry–Perot resonator mode at shorter wavelength have been observed when increasing the operation current. Between the mode hops, single longitudinal mode emission at around 405 nm is achieved with a full width at half-maximum of 0.03 nm. A linear redshift of the emission wavelength with increasing temperature of 0.019 nm/K was derived.

Polarization Instabilities in Vertical-Cavity Surface-Emitting Lasers

We report on experimental investigation of short-period InGaAs/InGaAlAs superlattice vertical cavity surface emitting lasers characteristics (VCSEL), including light-current-voltage characteristics, optical and radiofrequency spectra and polarization mode hopping between orthogonal modes. The observed polarization switching features is similar to what is observed in quantum well VCSEL. Future investigations will consider polarization-resolved optical and radiofrequency spectra, total intensity noise analysis of VCSEL biased near the polarization switching point.

Arm locking using laser frequency comb

The space-borne gravitational wave (GW) detectors, e.g., LISA, TaiJi, and TianQin, will open the window in the low-frequency regime (0.1 mHz to 1 Hz) to study the highly energetic cosmic events, such as coalescences and mergers of binary black holes and neutron stars. For the sake of successful observatory of GWs, the required strain sensitivity of the detector is approximately 10-21/Hz1/2 in the science band, 7 orders of magnitude better than the state of the art of the ultra-stable laser. Arm locking is therefore proposed to reduce the laser phase noise by a few orders of magnitude to relax the burden of time delay interferometry. During the past two decades, various schemes have been demonstrated by using single or dual arms between the spacecraft, with consideration of the gain, the nulls in the science band, and the frequency pulling characteristics, etc. In this work, we describe an updated version of single arm locking, and the noise amplification due to the nulls can be flexibly restricted with the help of optical frequency comb. We show that the laser phase noise can be divided by a specific factor with optical frequency comb as the bridge. The analytical results indicate that, the peaks in the science band have been greatly reduced. The performance of the noise suppression shows that the total noise after arm locking can well satisfy the requirement of time delay interferometry, even with the free-running laser source. When the laser source is pre-stabilized to a Fabry-Perot cavity or a Mach-Zehnder interferometer, the noise can reach the floor determined by the clock noise, the spacecraft motion, and the shot noise. We also estimate the frequency pulling characteristics of the updated single arm locking, and the results suggest that the pulling rate can be tolerated, without the risk of mode hopping. Arm locking will be a valuable solution for the noise reduction in the space-borne GW detectors. We demonstrate that, with the precise control of the returned laser phase noise, the noise amplification in the science band can be efficiently suppressed based on the updated single arm locking. Not only does our method allow the suppression of the peaks, the high gain, and low pulling rate, it can also serve for full year, without the potential risk of locking failure due to the arm length mismatch. We then discuss the unified demonstration of the updated single arm locking, where both the local and the returned laser phase noises can be tuned to generate the expected arm-locking sensor actually. Finally, the time-series simulations in Simulink have been carried out, and the results indicate a good agreement with the theory, showing that the presented method is reasonable and feasible. Our work could provide a back-up strategy for the arm locking in the future space-borne GW detectors.

High performance distributed feedback quantum cascade laser emitting at λ∼6.12um.

Distributed feedback quantum cascade lasers emitting at a wavelength of 6.12 µm are reported. Benefitted from the optimized materials epitaxy and the modified bound to continuum transition active region design along with three pairs of phonon scattering, high device performance is achieved. For a 2-mm-long, 8.4-µm-wide device, the threshold current is as low as 130 mA, the corresponding threshold current density is only 0.77 kA/cm2, and the optical output power is 69 mW at 20 °C in continuous wave mode. The temperature of continuous wave operation can reach 100 °C, where the optical output power is still more than 8 mW. In addition, it maintains a stable single mode operation from 20 to 100 °C without mode hopping, corresponding to a total wavelength shift of 41 nm. Such low-threshold quantum cascade lasers are highly beneficial to portable and highly integrated system sensor applications.

Speckle-displacement-based wavemeter for mode-hop and side-mode detection.

A speckle-displacement-based wavemeter is combined with a spatial-fundamental-mode-pass filter to eliminate the influence of multimode operation on the directionality of the resulting output from a distributed Bragg reflector (DBR) tapered laser. The proposed setup is characterized theoretically and experimentally, and detections of mode hops and side-mode suppression ratios (SMSRs) in the optical output are demonstrated. The laser illuminates a rough surface at an oblique angle, and a camera observes the corresponding speckle pattern from an almost identical back-scattering direction. As the wavelength of the laser shifts, the speckle pattern responds with a corresponding displacement, which is approximately linear with respect to the shift within the detection area. The wavemeter tracks continuously the shifts of the speckles pattern by tracking the peak of the covariance function of sequentially acquired images. In this way, the speckle-displacement-based wavemeter achieves a spectral resolution of 10.4 MHz. Mode hops in the laser do not cause any impeding decorrelation of the speckle patterns. Interestingly, the actual SMSR is related to the peak height and width of the absolute covariance function. A wavemeter, which is capable of measuring wavelengths, mode hops, and SMSRs, is highly useful for spectroscopy, quantum optics, nonlinear frequency conversion, and other applications requiring stable single-frequency laser light, especially when using diode lasers.

Compactly Efficient CW 3 to 4.5 μm Wavelength Tunable Mid-Infrared Laser in Optically Pumped Semiconductor Laser With Intracavity OPO

An efficient continuous-wave (CW) mid-infrared laser with a tunable wavelength range from 3 to 4.5 μm is demonstrated by using intracavity optical parametric oscillator (ICOPO). An optically pumped semiconductor laser (OPSL) is applied as the parent pumped laser gain medium to eliminate self-modulation effect caused by Nd-doped gain medium with ICOPO for fulfilling true CW emissions. Simultaneously, the OPSL gain chip with flip-chip packaging method can avoid mode hopping resulted from internal heat spreader etalon when strong depletion in ICOPO was achieved. The diode pump criteria of the OPSL is first investigated to prevent thermal roll over. By observing the spectral and temporal dynamics of the parent pumped OPSL with and without OPO operation, the true CW and mode-hop-free quality of the laser is verified. A low diode pump threshold of 2.67 W is obtained for the mid-IR idler emission and a total extracted idler power is 250 mW at a diode pump power of 8 W. Finally, the wavelength tunability is confirmed by adjusting the horizontal location of the multiple channel MgO:PPLN crystal, which indicated the feasibility of practical application in such a compact configuration.

Frequency-stabilized Brillouin random fiber laser enabled by self-inscribed transient population grating.

A frequency-stabilized Brillouin random fiber laser (BRFL) realized by a self-inscribed transient population grating (TPG) is proposed and demonstrated for the first time, to the best of our knowledge. The TPG is formed via the redistribution of the population in erbium-doped fibers (EDFs) by bidirectionally injected phonon-controlled random laser beams. Long-lifetime metastable ion states in EDFs basically prolonged the time dynamics of a stimulated Brillouin scattering (SBS) laser up to milliseconds. Consequently, significant random modes are suppressed with low relative intensity noise, owing to reduced mode hopping in a Stokes random laser, hence one dominating lasing mode at milliseconds of lifetime is established from the competition of numerous random modes, which is proved theoretically and experimentally via TPG.

Highly sensitive and stable fiber-laser pressure-sensing system based on an unequal-arm Mach-Zehnder cascaded with a Sagnac structure

We propose, analyze, and experimentally demonstrate a highly sensitive and stable fiber-laser pressure-sensing system based on an unequal-arm Mach-Zehnder interferometer (MZI) merged with a Sagnac interferometer (SI). The unequal-arm MZI structure consists essentially of two single-mode fibers of different lengths, and the SI structure uses a section of polarization-maintaining fiber, which acts as a sensing head. Moreover, in the proposed sensing system, the MZI and SI structures both act as a cascade filter. Given the asymmetric MZI and cascade filter structure laser mode hopping is effectively suppressed, so minor variations in environmental parameters cause a stable shift of the interference spectrum. The experimental results show that the proposed system can be used to make accurate pressure measurements. The −3 dB linewidth of the reflection interference peak produced by the proposed system is less than 0.02 nm, and the signal-to-noise ratio (SNR) can exceed 45 dB. With a 1-m-long sensor head, the proposed system provides a pressure sensitivity of 29.275 nm/MPa, and the interference spectrum of the sensing system fluctuates less than ±0.02 nm over 1 h. The proposed sensing system thus offers the attractive characteristics of good sensing linearity and stability, high SNR, and high sensitivity.

Self-Mixing Signal Characteristics of Complex-Coupled Distributed-Feedback Terahertz Quantum-Cascade Lasers

Self-mixing interference (SMI) in terahertz quantum cascade lasers (THz QCLs) is one of the significant approaches for coherent THz imaging and sensing techniques. Here, the output characteristics of SMI in distributed feedback (DFB) THz QCLs from the index-to the gain-coupling regimes are studied using the coupled wave theory and the multi-mode rate equation method. A mode hopping phenomenon is found to occur when the DFB coupling factor changes from index-coupling to gain-coupling, and the characteristics of the self-mixing signals of DFB-QCLs change greatly with this mode hopping. With the modulus of the coupling factor fixed and its argument varied from 0 to π/2, an extreme point of the self-mixing frequency and power signals of DFB-QCLs is found at π/9 due to the mode hopping. For index-coupling dominated DFB-QCLs, both the varying ranges of the self-mixing frequency signals and amplitudes of power signals increase with increasing DFB coupling factor argument. For gain-coupling dominated DFB-QCLs, with increasing argument value, the amplitude of the self-mixing power signal increases, but the varying range of the self-mixing frequency signal decreases. With the argument of the coupling factor fixed, we also found that the varying ranges of the self-mixing frequency signals decrease with increasing modulus for both index-coupling dominated and gain-coupling dominated DFB-QCLs. For index-coupling dominated DFB-QCLs, the amplitudes of the self-mixing power signals decrease with increasing modulus; however, the amplitudes of the self-mixing power signals of gain-coupling dominated DFB-QCLs increase. With the argument of the coupling factor fixed, for index-coupling dominated DFB-QCLs, we found that the varying ranges of the self-mixing frequency signals and amplitudes of power signals decrease with the increasing modulus. For gain-coupling dominated DFB-QCLs, with the coupling factor modulus increasing, the varying ranges of the self-mixing frequency signals decrease, however, the amplitudes of the self-mixing power signals increase. These results may help with the application of DFB-QCLs to self-mixing interferometers.

Terahertz imaging with self-pulsations in quantum cascade lasers under optical feedback

The phenomenon of self-pulsation (SP) in terahertz (THz) quantum cascade lasers (QCLs) due to optical feedback was reported recently. In this Letter, we propose a THz imaging modality using the SP phenomenon in a THz QCL. We explore the theoretical oscillation properties of the SP scheme and demonstrate its suitability to perform imaging experimentally. The SP imaging scheme operates in self-detection mode, eliminating the need for an external detector. Moreover, the scheme requires only a fixed current, meaning that one can avoid many of the pitfalls associated with high temperature operation of THz QCLs, including frequency chirp and mode hops caused by sweeping the laser current. This also means that one is free to locate the operating point at the maximum power, to produce the desired beam profile or for highest spectral purity, depending on the application. The SP imaging modality proposed in this work can be translated directly to high operating temperature THz QCLs.