Free-running Case Research Articles

Narrow-Linewidth 852-nm DBR-LD with Self-Injection Lock Based on High-Finesse Optical Cavity Filtering

Narrow-linewidth lasers have a high spectral purity, long coherent length, and low phase noise, so they have important applications in atomic clocks, precision measurement, and quantum computing. We inject a transmitted laser from a narrow-linewidth (∼15 kHz) flat-concave Fabry–Perot (F-P) cavity made from ultra-low expansion (ULE) optical glass into an 852 nm distributed Bragg reflector-type laser diode (DBR-LD), of which the comprehensive linewidth is 1.67 MHz for the free running case. With an increase in the self-injection power, the laser linewidth gradually narrows, and the injection locking current range gradually increases. The narrowest linewidth measured by the delayed frequency-shifted self-heterodyne (DFSSH) method is about 365 Hz, which is about 1/4500 of the linewidth for the free running case. Moreover, to characterize the laser phase noise, we use a detuned F-P cavity to measure the conversion signal from the laser phase noise to the intensity noise for both the free running case and the self-injection lock case. The laser phase noise for the self-injection lock case is significantly suppressed in the analysis frequency range of 0.1–10 MHz compared to the free running case. In particular, the phase noise is suppressed by more than 30 dB at an analysis frequency of 100 kHz.

Atmospheric impacts of chlorinated very short-lived substances over the recent past – Part 1: Stratospheric chlorine budget and the role of transport

Abstract. Impacts of chlorinated very short-lived substances (Cl-VSLS) on stratospheric chlorine budget over the first two decades of the 21st century are assessed using the Met Office’s Unified Model coupled to the United Kingdom Chemistry and Aerosol (UM-UKCA) chemistry–climate model; this constitutes the most up-to-date assessment and the first study to simulate Cl-VSLS impacts using a whole atmosphere chemistry–climate model. We examine the Cl-VSLS responses using a small ensemble of free-running simulations and two pairs of integrations where the meteorology was “nudged” to either ERA5 or ERA-Interim reanalysis. The stratospheric chlorine source gas injection due to Cl-VSLS estimated from the free-running integrations doubled from ∼40 ppt Cl injected into the stratosphere in 2000 to ∼80 ppt Cl injected in 2019. Combined with chlorine product gas injection, the integrations showed a total of ∼130 ppt Cl injected into the stratosphere in 2019 due to Cl-VSLS. The use of the nudged model significantly increased the abundance of Cl-VSLS simulated in the lower stratosphere relative to the free-running model. Averaged over 2010–2018, simulations nudged to ERAI-Interim and ERA5 showed 20 ppt (i.e. a factor of 2) and 10 ppt (i.e. ∼50 %) more Cl, respectively, in the tropical lower stratosphere at 20 km in the form of Cl-VSLS source gases compared to the free-running case. These differences can be explained by the corresponding differences in the speed of the large-scale circulation. The results illustrate the strong dependence of the simulated stratospheric Cl-VSLS levels on the model dynamical fields. In UM-UKCA, this corresponds to the choice between free-running versus nudged set-up, and to the reanalysis dataset used for nudging. Temporal changes in Cl-VSLS are found to have significantly impacted recent HCl and COCl2 trends in the model. In the tropical lower stratosphere, the inclusion of Cl-VSLS reduced the magnitude of the negative HCl and COCl2 trends (e.g. from ∼-8%(HCl)/decade and ∼-4 ppt(COCl2)/decade at ∼20 km to ∼-6%(HCl)/decade and ∼ −2 ppt(COCl2)/decade in the free running simulations) and gave rise to positive tropospheric trends in both tracers. In the tropics, both the free-running and nudged integrations with Cl-VSLS included compared much better to the observed trends from the ACE-FTS satellite record than the analogous simulations without Cl-VSLS. Since observed HCl trends provide information on the evolution of total stratospheric chlorine and, thus, the effectiveness of the Montreal Protocol, our results demonstrate that Cl-VSLS are a confounding factor in the interpretation of such data and should be factored into future analysis. Unlike the nudged model runs, the ensemble mean free-running integrations did not reproduce the hemispheric asymmetry in the observed mid-latitude HCl and COCl2 trends related to short-term dynamical variability. The individual ensemble members also showed a considerable spread of the diagnosed tracer trends, illustrating the role of natural interannual variability in modulating the diagnosed responses and the need for caution when interpreting both model and observed tracer trends derived over a relatively short time period.

Nanosecond pulsed single longitudinal mode diamond Raman laser in the 1.6 µm spectral region.

We demonstrate the first nanosecond pulsed single longitudinal mode (SLM) intracavity-pumped diamond Raman laser, to the best of our knowledge. The eye-safe coherent source at 1634 nm, which was converted from the actively Q-switched 1342 nm Nd:YVO4 laser, yielded 4.35 W of multimode average output power with a pulse duration of 6 ns and peak power of 29 kW. By exploiting the spatial hole burning free gain mechanism in the Raman media, stable SLM operation was observed at low output power (0.46 W) for the free-running case. Furthermore, by incorporating an etalon in the fundamental standing-wave cavity, the spectral linewidth of the fundamental field was suppressed substantially below the diamond Raman gain linewidth and slightly less than the free spectral range of the mm-scale Raman resonator. Thereby, a much higher SLM output power of 1.63 W was obtained with a pulse duration of ∼9 ns and a spectral linewidth of ∼77 MHz.

A linewidth narrowed diode laser for metastable Argon atom pumping

A volume Bragg grating-based external cavity diode laser array produced more than 100 W output power for metastable argon atoms pumping centered at 811.53 nm with a 0.15 nm spectral linewidth (FWHM). The device has only 4.6 % loss compared to the free-running case operating at full power and a spectral contrast of 40 dB. The emission spectrum can be precisely tuned to 811.53 nm and has an over 200 pm tuning range. Pump absorption was demonstrated by using a repetitively pulsed glow discharge operating with 620 Torr Ar/He mixture, and an average absorption of 24 % of the pump radiation was realized.

Suppression of Relative Intensity and Mode Partition Noises in Orthogonally Polarized Dual-Wavelength VCSEL

A synthesizer-free 28-GHz millimeter wave (MMW) carrier is generated by using orthogonally polarized dual-wavelength vertical-cavity surface-emitting laser (VCSEL) as the frontend transmitter. After equalizing the dual modes with an extremely weak power as low as −26 dBm (2.5 μW) at TM-mode polarization, the dual-wavelength power extinction of only 5.4 dB is achieved for optimizing the remote-node optical heterodyne of 28-GHz MMW carrier with a peak power of −59.8 dBm before antenna propagation with remaining similar relative intensity noise (RIN) as compared to the free-running case. Moreover, the mode partition noise (MPN) is increased less than 2 dBc/Hz by TM-mode injection at −26 dBm, whereas the TE-mode control results in unchanged noise spectrum as same as that obtained in free-running case even if the master power is increased to −18 dBm.

Tunable Violet Laser Diode System for Optical Wireless Communication

We report a tunable self-injection locked violet laser diode external cavity system exhibiting a continuous wavelength tunability of 5.15 nm (400.28 - 405.43 nm) with mean side-mode-suppression-ratio (SMSR) and linewidth of ~23 dB and ~190 pm, respectively. The effects of injection current and temperature indicate a robust system besides being cost-effective and straightforward. Moreover, a successful indoor on-off keying transmission at two different locked modes on a 0.4 m free space channel showed ~10 times improvement in the bit-error-rate (BER) with value ~8 × 10 -4 at 2 Gb/s, and better performance on 0.8 m channel length at 1.75 Gb/s compared to the free-running laser case. Our work is a potential step towards the realization of future high data capacity narrow-wavelength-spaced multiplexed optical wireless communication system wherein continuously tunable laser sources are expected to play a crucial role as transmitters.

RF Frequency Synthesizer Based on Self-Mode-Locked Multimode Lasers

Low-noise local oscillators are very important in modern communication systems. To realize a stable solution with a limited size, symmetric multimode multi-section semiconductor laser pairs are reported in this article for building a chip-level RF synthesizer. The symmetric laser structure utilizes a shared distributed Bragg reflector (DBR) and counterpropagating laser output with the help of an external feedback circuit to achieve self-mode-locking. The achieved phase noise performance is −92 dBc/Hz at a 10 kHz offset frequency for intermodal operation at 26.4 GHz using 5 locked modes, which is a 57 dB improvement compared to the free-running case. Moreover, an RF synthesizer covering frequencies of 11–13 GHz is realized based on the achieved mode-locked system using current tuning. The related phase noise of the laser pair beat-note output is −100 dBc/Hz at a 10 kHz offset of carrier frequencies of 11–13 GHz, which is 58 dB better than the phase noise of the beat-note output without mode-locking over 11–13 GHz.

Frequency and power stabilization of a terahertz quantum-cascade laser using near-infrared optical excitation.

We demonstrate a technique to simultaneously stabilize the frequency and output power of a terahertz quantum-cascade laser (QCL). This technique exploits frequency and power variations upon near-infrared illumination of the QCL with a diode laser. It does not require an external terahertz optical modulator. By locking the frequency to a molecular absorption line, we obtain a long-term (one-hour) linewidth of 260 kHz (full width at half maximum) and a root-mean-square power stability below 0.03%. With respect to the free-running case, this stabilization scheme improves the frequency stability by nearly two orders of magnitude and the power stability by a factor of three.

A 1.8 V 18.13 MHz Inverter-Based On-Chip RC Oscillator with Flicker Noise Suppression Using Logic Transition Voltage Feedback

An inverter-based on-chip resistor capacitor (RC) oscillator with logic transition voltage (LTV) tracking feedback for circuit delay compensation is presented. In order to achieve good frequency stability, the proposed technique considers the entire inverter chain as a comparator block and changes the LTV to control the oscillation frequency. Furthermore, the negative feedback structure also reduces low-frequency offset phase noise. With a 1.8 V supply and at room temperature, the suggested oscillator operates at 18.13 MHz, consuming 245.7 μ W. Compared to the free-running case, the proposed technique reduces phase noise by 7.7 dB and 5.45 dB at 100 Hz and 1 kHz, respectively. The measured phase noise values are −60.09 dBc/Hz at 1 kHz with a figure of merit (FOM) of 151.35 dB/Hz, and −106.27 dBc/Hz at 100 KHz with an FOM of 157.53 dBc/Hz. The proposed oscillator occupies 0.056 mm2 in a standard 0.18 μ m CMOS process.

Self-Injection Locking of a Distributed Feedback Laser Diode Using a High-Finesse Fabry-Perot Microcavity

Optical microcavities have been widely used in nonlinear optics, quantum optics, and laser technologies. Here we demonstrate the self-injection locking of a distributed feedback diode laser using home-made high-finesse Fabry-Perot microcavity. The Fabry-Perot microcavity is fabricated from an x-cut lithium niobate crystal with highly reflective coatings. Frequency pulling effect can be observed for a successful locking, and results in a single-longitudinal mode lasing with narrow linewidth. The lasing wavelength and output power are found robust to the laser-diode current and temperature variations, in comparison to the free-running case. We further characterize the laser linewidth with beat note measurement with a high-performance external cavity diode laser, with beat-note linewidth of 601.85 kHz. This results shows a new method for laser frequency stabilization in a simple setup, and may find applications in telecommunication and spectroscopy.

Self-forced stabilization of inter-modal oscillation in multi-section semiconductor lasers at X-band.

Forced oscillation based on self-injection locked phase-locked (SILPLL) opto-electronic oscillator (OEO) technique is the basis of highly stable and low phase noise oscillators to be used as a key component in modern communication and sensing systems. To avoid large-size modular OEO, inter-modal oscillation at X-band frequencies is achieved in a compact multi-mode multi-section semiconductor laser using InP foundry service. Self-forced oscillation technique of SILPLL is demonstrated, where a phase noise performance of -98 dBc/Hz is measured at 10 kHz offset carrier of 11 GHz, which is an improvement of 68 dB compared with the free-running condition, which corresponds with 600 times reduction in calculated timing jitters from the free-running case to reach 0.45 ps for over 1 kHz to 1 MHz bandwidth.

Frequency locking of semiconductor lasers to RF oscillators using hybrid-integrated opto-electronic oscillators with dispersive delay lines.

Direct frequency locking of lasers to RF oscillators has many applications such as high resolution optical frequency synthesis, coherent optical communication, spectroscopy, sensing, and imaging. Here we present a hybrid-integrated opto-electronic loop that directly frequency locks a semiconductor laser to an RF synthesized source using an opto-electronic oscillator with a dispersive optical delay line. Cascaded ring filters, operating near the resonance frequency, provide an enhanced chromatic dispersion with a compact footprint. The electronic chip is integrated in the GlobalFoundries 180 nm CMOS SOI technology and the photonic chip is integrated in the IME 180 nm SOI technology. A tracking range of 0.5 GHz is achieved while consuming 33 mW power. The proposed scheme is used to frequency lock a commercially available DFB laser, reducing the laser frequency fluctuations by an order of magnitude compared to the free-running case.

Spectral Narrowing and Stabilization of 10-W Broad-Area Laser With Fiber Pump Delivery System

The emission spectrum of a 10-W commercial broad-area laser operated at full power was narrowed and stabilized by coupling an off-axis portion of its far-field emission to a single-mode fiber Bragg grating. The feedback produced in this partial compound cavity configuration narrowed the FWHM linewidth of the laser by $7\times $ compared with the free-running case and stabilized its peak wavelength over a 7° temperature range. Furthermore, the feedback improved the spectral density of the emission by $2\times $ over a 1.5° temperature range and over most of the far-field emission angles, indicating a robust concept suitable for manufacturing. This proof-of-concept experiment can be translated into a fiber pump delivery system that can capture all the broad-area laser (BAL) output while providing feedback to improve the spectral and potentially spatial properties of BAL emission.

Evaluation of laser frequency offset locking using an electrical delay line.

Frequency offset locking between two Nd:YAG lasers is performed using frequency locking with an electrical delay line. The relative frequency instability of the offset locking is measured to be 3.5×10-12 for an averaging time of 1s, which is approximately 77 times smaller than that of the free-running case. The frequency instability of the frequency locking is compared to that of the phase locking between the two Nd:YAG lasers. Furthermore, a compact solid-state laser is frequency locked to an optical frequency comb with a frequency instability of 8.2×10-11 for an averaging time of 1s, which is improved by approximately 20 times, with respect to the free-running case. The offset-locking scheme using a delay line is useful for various applications including a research on quantum optics, interferometric measurements, and experiments involving laser cooling, such as an optical lattice clock.

Radio-over-fiber transmission using semiconductor laser under weak external light injection

Optical injection locking is a promising method to overcome the limitations exhibited by directly modulated distributed feedback lasers in analog radio-over-fiber applications. Here the effects of optical injection locking are investigated experimentally for relatively low injection ratio, which is favored for practical applications. The measured responses of optical injection locking with low injection ratio still show distinctive modulation characteristics, controllable by the frequency detuning and the injection ratios. At the higher positive detuning the resonance peak can extend to 22 GHz (limited only by the analyzer instrument), which is 4.2 times greater than in the free running case. Reduction in the noise level becomes better as the system moves towards negative detuning. The reduction reaches to 5.97 dB at the resonance peak for −20 GHz detuning and a −2.9 dB injection ratio. Moreover, reduction also can be realized in the low frequency regime 0–10 GHz. We also achieved a 9.1 dB spur free dynamic range improvement. The error vector magnitude requirement specification of 3.1 % for 64-QAM OFDMA can be satisfied by an injection locked laser with 5–10 dB enhancements, comparing to a free running laser even after transmission over different transmission distances.

Experimental Study of the Timing Jitter of a Passively Mode-Locked External-Cavity Semiconductor Laser Subject to Repetition Rate Transitions and Optical Feedback

We experimentally investigate the timing jitter (TJ) of a passively mode-locked external-cavity diode laser. Variation of the gain current and the absorber reverse bias voltage allows transitions from fundamental mode-locking up to seventh harmonic mode-locking. Hereby, a reduction of the TJ as a function of the harmonic mode-locking order is found. Furthermore, the application of optical feedback results in an additional reduction of TJ for almost the whole investigated operation range. In particular, the reduction increases with harmonic mode-locking order. The highest observed reduction of TJ amounts to a factor of 10 as compared with the free-running case which corresponds to a repetition-rate linewidth reduction by a factor of 100.

30% improvement in absorption spectroscopy detectivity achieved by the detuned loading of a quantum cascade laser.

We perform a direct absorption spectroscopy experiment of carbon monoxide at 2193 cm(-1) by exploring the detectivity improvement potential of an intensity noise (IN)-reduced distributed feedback (DFB) quantum cascade laser. This was achieved by a detuned loading approach via a short, phase-sensitive optical feedback cavity. Under optimum IN reduction conditions, we obtain an improvement in signal-to-noise ratio from 733 to 1048, which transfers into a detection limit improvement from 1.2 ppm to 840 ppb. Therefore, we achieve a 30% lower detection limit, with the IN reduced when compared to the free-running case.

Self-injected semiconductor distributed feedback lasers for frequency chirp stabilization

It is well known that semiconductor distributed feedback lasers (DFB) are key devices for optical communications. However direct modulation applications are limited by the frequency chirp induced by current modulation. We demonstrate that a proper external control laser operation leads to chirp-to-power ratio (CPR) stabilization over a wide range of modulation frequencies as compared to the free-running case. Under experimentally selected optical feedback conditions, the CPR decreases significantly in the adiabatic regime from about 650 MHz/mW in the solitary case down to 65 MHz/mW. Experimental results are also confirmed by numerical investigations based on the transfer matrix method. Simulations point out the possible optimization of the CPR in the adiabatic regime by considering a judicious cavity design in conjunction with a proper external control. These results demonstrate important routes for improving the transmission performance in optical telecommunication systems.

Optically injection-locked photonic crystal laser with >67 GHz modulation bandwidth

A buried heterostructure photonic crystal laser with a record-high direct modulation bandwidth of more than 67 GHz has been demonstrated using optical injection-locking. The resonance frequency of an injection-locked photonic crystal laser has been increased from 8 GHz at the free-running case to 64.4 GHz.

An 80-W Laser Diode Array with 0.1 nm Linewidth for Rubidium Vapor Laser Pumping

The spectral linewidth of a 64-emitter laser-diode array is effectively suppressed by using a volume Bragg grating (VBG) based external cavity. At a maximal driven current of 90 A, the device produces a cw output of 80 W with 1.2 W/A slope efficiency and 0.1 nm spectral linewidth (FWHM) centered at 780nm. The power extraction efficiency reaches 90% as compared with the free running case. The central wavelength of the narrowed spectrum is tuned over a 0.3 nm range by adjusting the VBG's temperature. The absorption of 45% laser radiation by a 5-mm-long rubidium vapor cell with 150 Torr ethane and 450 Torr helium at 383 K is demonstrated.