Relaxation Oscillation Resonance Research Articles

Influence of the pump scheme on the output power and the intensity noise of a single-frequency continuous-wave laser.

The influence of the pump scheme on the intensity noise of the single-frequency continuous-wave (CW) laser is investigated in this paper, which is implemented in a single-frequency CW Nd:YVO4 1064 nm laser by comparing the traditional 808 nm pumping scheme (TPS) to the direct 888 nm pumping scheme (DPS). Under the conditions that the lasers with TPS and DPS have the same cavity structure and the cavity mirrors, as well as the same operation state including the thermal lens of the laser crystals and the mode-matching between the pump laser mode and the laser cavity mode at the laser crystals, the output power of the laser with DPS is up-to 32.0 W, which is far higher than that of 21.1 W for the laser with TPS. However, the intensity noise of the DPS laser including resonant relaxation oscillation (RRO) frequency of 809 kHz, RRO peak amplitude of 31.6 dB/Hz above the shot noise level (SNL) and the SNL cutoff frequency of 4.2 MHz, respectively, is also higher than that of 606 kHz, 20.4 dB/Hz and 2.4 MHz for the TPS laser. After further analyses, we find that the laser crystal with high doping concentration and long optical length is employed for DPS laser in order to improve the pump laser absorption efficiency, which can simultaneously increase the dipole coupling between the active atoms and the laser cavity, and then results in a high RRO frequency with a large amplitude peak as well as a high SNL cutoff frequency of the laser.

Intensity noise suppression of a high-power single-frequency CW laser by controlling the stimulated emission rate.

The intensity noise of a high-power single-frequency continuous-wave (CW) laser is very harmful for applications in precise measurements and quantum communication. By simply lengthening the length of a laser resonator to decrease the stimulated emission rate of the laser, the coupling strength of all noise sources in the resonant laser field will be reduced, and thus the intensity noise of the output laser will be prominently suppressed. Based on theoretical analyses of the laser noise spectra, we experimentally implement a low-noise high-power single-frequency laser with a 1050 mm long resonator. With the assistance of an intracavity imaging system and nonlinear second-harmonic generation, the amplitude of the resonant relaxation oscillation peak and the shot-noise level (SNL) cutoff frequency are successfully reduced to 8.6 dB/Hz above SNL and 1.0 MHz, respectively, under the output power of 16 W. The work provides an effective way to develop a high-quality laser with high output power and low intensity noise.

Semiconductor Fano Lasers

In this paper, a new type of semiconductor laser based on Fano interference is described. One of the laser mirrors relies on the interference between the continuum of waveguide modes and a side-coupled nanocavity, leading to a narrow-band mirror that provides the Fano laser with unique characteristics. In addition to being truly single-mode, the laser can be modulated through the mirror at frequencies far exceeding the relaxation oscillation resonance. Furthermore, nonlinearities in the nanocavity can be used to implement a saturable mirror, leading to passive pulse generation with repetition frequencies in the gigahertz range. This paper reviews the theory of Fano lasers and the current experimental status. Experimentally, the Fano laser concept is demonstrated using a photonic crystal platform with quantum dot active material. Both continuous wave operation and self-pulsing is observed for optically pumped lasers operating at room temperature.

Quantum control of phase fluctuations in semiconductor lasers

Few laser systems allow access to the light-emitter interaction as versatile and direct as that afforded by semiconductor lasers. Such a level of access can be exploited for the control of the coherence and dynamic properties of the laser. Here, we demonstrate, theoretically and experimentally, the reduction of the quantum phase noise of a semiconductor laser through the direct control of the spontaneous emission into the laser mode, exercised via the precise and deterministic manipulation of the optical mode's spatial field distribution. Central to the approach is the recognition of the intimate interplay between spontaneous emission and optical loss. A method of leveraging and "walking" this fine balance to its limit is described. As a result, some two orders of magnitude reduction in quantum noise over the state of the art in semiconductor lasers, corresponding to a minimum linewidth of [Formula: see text], is demonstrated. Further implications, including an additional order-of-magnitude enhancement in effective coherence by way of control of the relaxation oscillation resonance frequency and enhancement of the intrinsic immunity to optical feedback, highlight the potential of the proposed concept for next-generation, integrated coherent systems.

Investigation about the influence of longitudinal-mode structure of the laser on the relative intensity noise properties

The influence of the longitudinal-mode structure (LMS) of the laser on the relative intensity noise (RIN) properties was investigated in this paper after an all-solid-state continuous-wave (CW) single-frequency 1064 nm laser with output power of 50.3 W was achieved. The LMS of the laser was manipulated by controlling the temperature of the nonlinear lithium triborate (LBO) crystal deliberately introduced to the resonator. When the laser worked with single-longitudinal-mode (SLM) operation, the stable RIN spectrum was observed and measured. Moreover, with the decrease of the nonlinear conversion efficiency (NCE), the frequency and amplitude of the resonant-relaxation oscillation (RRO) peak regularly shifted to the higher frequencies and increased, respectively. However, with further decrease of the NCE, the laser began to work with the multi-longitudinal-mode (MLM) or mode-hopping operation and the unstable RIN spectra of the laser were both observed not only at low frequencies but also at high frequencies. Once the NCE was moved away, the MLM or mode-hopping can only enhance the fluctuation of the laser RIN spectrum at the low frequencies (lower than the frequency of RRO). The experimental results directly revealed the relationship between the LMS of the laser and the RIN spectra, and confirmed definitely that the key to achieve a stable high power laser with low intensity noise was to realize single-frequency operation of the laser with free MLM and mode-hopping.

White-Chaos Radar With Enhanced Range Resolution and Anti-Jamming Capability

Laser chaos is promising for constructing high range-resolution and anti-jamming radars, whereas the range resolution and anti-jamming capability are unfortunately limited by laser relaxation oscillation and external-cavity resonance, respectively. Here, we theoretically propose using a broadband white chaos generated by optical heterodyne of two chaotic external-cavity semiconductor lasers as source signal to construct a radar system with enhanced range resolution and anti-jamming capability. The white chaos not only has a white-noise-like wide spectrum without relaxation oscillation, but also has no signature of external-cavity resonance. Benefitting from these merits, the proposed white-chaos radar shows a marvelous unambiguous detection performance with subcentimeter range resolution. Moreover, the elimination of external-cavity resonance enables the randomness improvement of source signal, and resultantly, the anti-jamming capability is enhanced as well.

Minimal-post-processing 320-Gbps true random bit generation using physical white chaos.

Chaotic external-cavity semiconductor laser (ECL) is a promising entropy source for generation of high-speed physical random bits or digital keys. The rate and randomness is unfortunately limited by laser relaxation oscillation and external-cavity resonance, and is usually improved by complicated post processing. Here, we propose using a physical broadband white chaos generated by optical heterodyning of two ECLs as entropy source to construct high-speed random bit generation (RBG) with minimal post processing. The optical heterodyne chaos not only has a white spectrum without signature of relaxation oscillation and external-cavity resonance but also has a symmetric amplitude distribution. Thus, after quantization with a multi-bit analog-digital-convertor (ADC), random bits can be obtained by extracting several least significant bits (LSBs) without any other processing. In experiments, a white chaos with a 3-dB bandwidth of 16.7 GHz is generated. Its entropy rate is estimated as 16 Gbps by single-bit quantization which means a spectrum efficiency of 96%. With quantization using an 8-bit ADC, 320-Gbps physical RBG is achieved by directly extracting 4 LSBs at 80-GHz sampling rate.

УМЕНЬШЕНИЕ РЕЗОНАНСА РЕЛАКСАЦИОННЫХ АВТОКОЛЕБАНИЙ КОЛЕСНЫХ ТОРМОЗНЫХ МЕХАНИЗМОВ ЛЕСОВОЗНЫХ АВТОМОБИЛЕЙ

УМЕНЬШЕНИЕ РЕЗОНАНСА РЕЛАКСАЦИОННЫХ АВТОКОЛЕБАНИЙ КОЛЕСНЫХ ТОРМОЗНЫХ МЕХАНИЗМОВ ЛЕСОВОЗНЫХ АВТОМОБИЛЕЙ

Intensity noise manipulation of a single-frequency laser with high output power by intracavity nonlinear loss.

The influences of intracavity nonlinear losses on the intensity noise of output lasers are theoretically and experimentally investigated with an all-solid-state single-frequency laser with high gain. By means of tuning the temperature of a nonlinear crystal deliberately placed inside the laser resonator, the intracavity nonlinear loss of the laser is controlled. The dependence of the frequency and amplitude of the resonant relaxation oscillation peak on the nonlinear loss is studied in detail for both fundamental-wave (FW) and second-harmonic-wave (SHW). We find that, by controlling the temperature of the nonlinear crystal, the intensity noises of the laser can be transferred between FW and SHW. The theoretical predictions are in good agreement with the experimental measurements. The obtained results can be applied to manipulate and suppress the laser noises.

Significant intensity noise suppression of single-frequency fiber laser via cascading semiconductor optical amplifier

Significant suppression of the intensity noise of single-frequency fiber laser is demonstrated with a cascading semiconductor optical amplifier (SOA). Based on the nonlinear amplification dynamics of the SOA, intensity noise reduction would take place in every transmission of the laser signal. By cascading two SOAs, a maximum noise suppression of 30 dB at around the resonant relaxation oscillation (RRO) frequency as well as a suppression bandwidth of up to 50 MHz is realized. Moreover, the RRO peak is restricted to a significant narrow frequency band, outside of which the laser noise approaches the noise floor of the measurement. The remarkable amplified spontaneous emission (ASE) introduced by the SOA is entirely filtered out with a fiber Bragg grating (FBG). Furthermore, no noticeable degradation of laser frequency noise has been observed.

The ASE noise of a Yb3+-doped phosphate fiber single-frequency laser at 1083 nm

A thorough investigation of the effect of amplified spontaneous emission (ASE) on the noise characteristics of a heavily Yb-doped phosphate fiber single-frequency laser at 1083 nm was made. Both the intensity noise and the frequency noise were measured and analyzed by introducing a band pass filter (BPF) into the fiber laser. For the intensity noise, it was found that the ASE noise is present at frequencies beyond the resonant relaxation oscillation (RRO) and until 6 MHz at low pump intensity, while it is diminished in the high power regime. Under a pump power of 42 mW, a maximum reduction of over 3 dB of the relative intensity noise (RIN) was observed with the help of the BPF. As for the frequency noise, a transition of the dominating noise sources from ASE noise in the low pump intensity condition to pump noise in the high pump intensity condition was observed. In the low power condition, more than 7 dB of the ASE noise was found to add to the frequency noise spectrum. It is believed that the obtained results will be helpful in understanding and optimizing the noise characteristics of this type of fiber laser.

Intensity noise of erbium-doped fiber laser based on full quantum theory

Traditionally, by means of full quantum theory, we present the intensity noise transfer function of an Er-doped fiber laser, on the basis of which we analyze the spectrum of the intensity noise. Our theoretical results are in agreement with the existing experiment results. This model explains not only how the noise is produced, but also how the spontaneous emission and dipole fluctuations have an effect on the output noise, which cannot be explained via rate equation theory. We analyze the physical sources of various contributions to the noise spectrum as well. The simulation results show that the noise of the Er-doped fiber laser mainly consists of the vacuum noise resulting from the output coupling, dipole fluctuation noise, the pump source intensity noise, and the spontaneous emission from the upper level to the ground level, which provides the theoretical basis for noise suppression. Compared to the solid laser, the Er-doped fiber laser shows lower resonant relaxation oscillation frequency.

Single-SectionFabry-Perot Mode-Locked Semiconductor Lasers

We present a review of the theoretical models and experimental verification of the single-section Fabry-Perot mode-locked semiconductor lasers based on multiple-spatial-mode (MSM) coupling. The mode-locked operation at the repetition rates of 40 GHz and higher and the pulse width of a few picoseconds are confirmed by the intensity autocorrelation, the fast photo detection and RF spectrum, and the optical spectral interference measurement of ultrafast pulse. The spatial mode coupling theory of single-section Fabry-Perot mode-locked semiconductor lasers is also reviewed, and the results are compared with the experimental observations. The small signal modulation response of these lasers, which exhibits high-frequency responses well beyond the relaxation oscillation resonance limit, is also modeled theoretically, and the simulation is verified by the experimental measurements.

High-speed electroabsorption modulation of composite-resonator vertical-cavity lasers

The high-speed characteristics of a composite-resonator vertical-cavity laser (CRVCL) under both small- and large-signal electroabsorption modulation are investigated. The maximum -3 dB modulation bandwidth of 12.5 GHz is measured. The theoretical modulation response of the CRVCL is derived from the standard rate equations, and the qualitative form is in agreement with the measured results. The CRVCL under electroabsorption modulation exhibits a promising potential to extend the modulation bandwidth beyond that of a conventional VCSEL. However, the CRVCL under large-signal electroabsorption modulation is limited to low data rates, and the large relaxation oscillation resonance must be suppressed for higher-speed digital-optical link applications.

Frequency Response Enhancement of Optical Injection-Locked Lasers

The modulation response of injection-locked lasers has been carefully analyzed, theoretically and experimentally, with a focus on the strong optical injection regime. We derive closed-form solutions to the relaxation oscillation (resonance) frequency and damping term, as well as the low-frequency damping term, and discuss design rules for maximizing resonance frequency and broadband performance. A phasor model is described in order to better explain the enhancement of the resonance frequency. Experimental curves match closely to theory. Record resonance frequency of 72 GHz and broadband results are shown.

High-speed dynamics of GaAsSb vertical-cavity lasers

The intensity noise spectrum is measured for antimonide based 1.3 μm vertical-cavity surface emitting lasers. Analysis of the noise spectra indicates a modulation bandwidth of 5.7 GHz with an intrinsic maximum frequency response that exceeds 28 GHz. A discontinuity in the relaxation oscillation resonance frequency is observed, which suggests that the lasing behavior at threshold is assisted by nonlinearities in the gain medium.

High-frequency modulation without the relaxation oscillation resonance in quantum cascade lasers

Quantum cascade (QC) lasers, based on intersubband transitions in semiconductor quantum wells, are characterized by ultrafast (picosecond) carrier lifetimes. An important consequence of this unique property is the expected absence of relaxation oscillations in the transient response of these devices. Here, we discuss and experimentally verify this prediction by measuring the modulation response of several 8-μm-QC lasers, properly processed and packaged for high-speed operation, up to 10 GHz.

Frequency and Intensity Noise of Single Frequency Fiber Bragg Grating Lasers

The relative intensity (RIN) and optical frequency (νrms) noise of Er-doped fiber Bragg grating lasers are investigated theoretically and experimentally. For a 1480-nm pumped fiber distributed feedback (DFB) laser with 170-μW output power we observe a typical RIN floor of −118 dBHz/Hz for frequencies above 10 Hz, with a strong relaxation oscillation resonance peak at 222 kHz reaching −77dBHz/Hz.νrms ranges from 26dBHz/Hz at 1 kHz to 2dBHz/Hz at 1 MHz, with a sharp peak at the relaxation oscillation frequency. Below 1 kHz 1/f noise is observed in νrms. Negative feedback to the pump laser from the fiber laser output power is found to eliminate the relaxation oscillation noise peaks both from RIN and νrms spectra. By comparing measured intensity and frequency fluctuations at the relaxation oscillation frequency, we estimate the linewidth broadening factor of two investigated lasers to be 2.1 and 3.6, as compared to typically between 3 and 7 for semiconductor lasers.

Stability of an Er–Yb-doped fiber distributed-feedback laser with external reflections

The maximum tolerable amplitude backreflection coefficient r(ext,c) into a fiber distributed-feedback laser before the onset of instability has been investigated. r(ext,c) was found to decrease with increasing external cavity lengths up to ~320 m and to be proportional to the relative linewidth of the relaxation oscillation resonance. The tolerable length of Rayleigh backscattering standard telecom fiber was found to be 135-200 m. An observed degradation in the laser stability and slope efficiency at low pump powers is believed to be due to UV-induced saturable absorbers.

The modulation response of a semiconductor laser amplifier

We present a theoretical analysis of the modulation response of a semiconductor laser amplifier. We find a resonance behavior similar to the well-known relaxation oscillation resonance found in semiconductor lasers, but of a different physical origin. The role of the waveguide (scattering) loss is investigated in detail and is shown to influence the qualitative behavior of the response. In particular, it is found that a certain amount of waveguide loss may be beneficial in some cases. Finally, the role of the microwave propagation of the modulation signals is investigated and different feeding schemes are analyzed. The nonlinear transparent waveguide, i.e., an amplifier saturated to the point where the stimulated emission balances the internal losses, is shown to be analytically solvable and is a convenient vehicle for gaining qualitative understanding of the dynamics of modulated semiconductor optical amplifiers.