Low-frequency Intensity Noise Research Articles

Simultaneous achievement of power boost and low-frequency intensity noise suppression in a bidirectional pumping fiber amplifier based on saturated even-distribution gain.

An optimized bidirectional pumping fiber amplifier is demonstrated to achieve low-frequency intensity noise suppression and effective power enhancement simultaneously. Based on the concept analysis of the gain saturation effect, the influence of input signal power and pump power on intensity noise suppression is investigated and optimized systematically. Further combining with the optimization of the pumping configuration to achieve the even-distribution gain, the relative intensity noise (RIN) of 1083 nm single-frequency fiber laser (SFFL) is suppressed with 9.1 dB in the frequency range below 10 kHz. Additionally, the laser power is boosted from 10.97 dBm to 25.02 dBm, and a power instability of ±0.31% is realized. This technology has contributed to simultaneously improving the power and noise performance of the 1083 nm SFFL, which can be applied to a multi-channel helium (He) optically pumping magnetometer. Furthermore, this technique has broken the mindset that power amplification of the conventional fiber amplifiers will inevitably cause the degradation of intensity noise property, and provided a valuable guidance for the development of high-performance SFFLs.

Phase-shifted demodulation scheme for fiber-optic interferometric sensors with combined waveform phase modulation

We propose and demonstrate a demodulation scheme for interferometric optical fiber sensing using combined waveform phase modulation. The method is based on a designed combined waveform and time-division separation technology, which moves the target signal to the sidebands of the carrier signal and eliminates the influence of residual direct current (DC) term and low-frequency intensity noise by filtering the detector signal directly. The effect of the modulation-to-target ratio (MTR) defined as the ratio of the modulation frequency to the target frequency on the demodulation results is also discussed. Both simulations and experiments verify the ability to obtain a pair of quadrature signals. The experimental results indicate that the phase resolution of the system is 1.1 × 10−6 rad/√Hz@2 kHz when the MTR is 625, and 5 × 10−6 rad/√Hz@2 kHz when the MTR is 25, while demonstrating excellent demodulation consistency. This work not only has the advantages of carrier modulation methods, but also does not need to generate analog signals for mixing, which provides guidance for high-sensitivity phase demodulation technology.

Noise induced in optical fibers by double Rayleigh scattering of a laser with a 1/f^ν frequency noise

We study, theoretically and experimentally, intensity noise induced by double Rayleigh scattering in long optical fibers. The results of the theoretical model are compared to experimental results performed with a high-coherence-length laser with a frequency noise spectrum that is dominated by 1/fν noise. Excellent quantitative agreement between theoretical and experimental RF spectra were obtained for frequencies as low as 10 Hz and for fiber lengths between 4 and 45 km. Strong low-frequency intensity noise that is induced by 1/fν frequency noise of the laser may limit the performance of interferometric fiber optic sensors that require high-coherence-length lasers. The intensity noise due to double Rayleigh backscattering can be suppressed by reducing the coherence length of the laser. Therefore, the intensity noise has a complex and non-monotonic dependence on the 1/fν frequency noise amplitude of the laser. Stimulated Brillouin scattering will add a significant noise for input powers greater than about 7 mW for a 30 km length fiber.

Suppression of the low frequency intensity noise of a single-frequency Yb3+-doped phosphate fiber laser at 1083 nm

The suppression of the low frequency intensity noise of a 1083 nm single-frequency Yb3+-doped phosphate fiber laser is reported. The noise suppression scheme is to use a liquid crystal device as a variable attenuator to modulate the laser power to close to the desired level. The achieved long term (12 h) laser instability is less than 0.2%, while the relative intensity noise (RIN) has been decreased to lower than −140 dB Hz−1 at frequencies from 250 Hz to 1 kHz. Moreover, the laser linewidth value remains the same as before suppression, while the degree of polarization (DOP) declined slightly.

Low-frequency transmitted intensity noise induced by stimulated Brillouin scattering in optical fibers

We study theoretically and experimentally the spectral properties of low-frequency transmitted intensity noise induced by stimulated Brillouin scattering in optical fibers. In fibers with a length of 25 km the Brillouin scattering induces transmitted intensity noise with a bandwidth on the order of tens of kHz. The power spectral density of the noise can be stronger than the shot noise in the photo-detector even when the optical power is significantly lower than the Brillouin threshold. The low-frequency transmitted intensity noise is caused due to depletion of the pump wave by the stochastic Brillouin wave. Since pump depletion occurs over a long distance, noise with a narrow bandwidth is generated in the transmitted wave. When the pump power is high enough, the spectrum of the induced noise contains features such as hole at low frequencies and ripples. Good quantitative agreement between theory and experiments is obtained. Low-frequency intensity noise induced by Brillouin scattering may limit the generation of ultra-low noise signals in optoelectronic oscillators and may limit the transfer of ultra-low noise signals in fibers.

Polarization-resolved intensity noise in erbium-ytterbium codoped fiber lasers

The intensity-noise properties and polarization dynamics of a two-polarization Er:Yb codoped fiber laser are investigated and discussed. For a laser without polarization control, the low-frequency relative intensity noise reaches a minimum of approximately −90 dB/Hz. Polarization-sensitive detection reveals that the low-frequency intensity noise is influenced by a strong resonance associated with anticorrelated intensity fluctuations of the orthogonal linearly polarized modes. The frequency of the resonance varies linearly with laser power and leads to an increase in low-frequency noise of more than 45 dB for the polarized output of the laser.

Intensity noise suppression in fibre DFB laser using gain saturated SOA

Experimental results of a simple, all-optical technique to reduce low-frequency intensity noise in an Er–Yb codoped fibre distributed feedback (DFB) laser are reported. This method utilises the gain dynamics of a saturated semiconductor optical amplifier (SOA) to produce a reduction of 30 dB in the relaxation oscillation noise component of the fibre laser.

Intensity noise characteristics of erbium-doped distributed-feedback fiber lasers

We present an experimental and theoretical investigation into the low-frequency intensity noise characteristics of erbium-doped distributed feedback (DFB) fiber lasers. The intensity noise characteristics of six nonidentical erbium-doped DFB fiber lasers are presented along with the characteristics of the grating and doped fibers. An analytical model has been used to predict the intensity noise generated in a linear fiber laser and explain the observed noise characteristics. Overall we find good agreement between our analytical model and observations. In particular, we find the intensity noise at frequencies close to the relaxation oscillation frequency significantly elevated due to excess noise from either spontaneous emission or cavity loss modulation. These results can be used to optimize the fiber laser design for sensor applications.

Effects of Pauli blocking on semiconductor laser intensity and phase noise spectra

In this paper we derive the intensity and phase noise spectra of a single-mode semiconductor laser on the basis of an operatorial Langevin set of equations that includes the dynamics of the microscopic variables, i.e., the carrier polarization and their distribution over the k states. In particular we take into account the fact that the carriers pumped into the active layer are subject to the blocking determined by Pauli exclusion principle. We demonstrate that, due to the fastness of the carrier scattering and polarization dephasing processes, the noise spectra can be determined on the basis of a macroscopic linearized set of three equations for the two quadrature components of the laser intensity and the total carrier number. A formal comparison with the paradigmatic results of [Yamamoto et al. Phys. Rev. A 34, 4025 (1986)] allows to deduce that the only essential difference arises from Pauli-induced pump blocking, which has the effect of increasing the low-frequency branch of the intensity noise spectrum. We demonstrate that, even for very small amounts of pump blocking, the low-frequency intensity noise steeply rises with the stationary value of the carrier density in the active layer, which depends on a great number of parameters. This result can explain the erratic behavior of the experimental findings and their discrepancy with the standard theoretical predictions.

Suppression of the intensity noise of a laser-diode-pumped single-frequency ring Nd:YVO_4–KTP green laser by optoelectronic feedback

We investigate the different characteristics of the intensity noise of a laser-diode-pumped single-frequency ring Nd:YVO (4) laser and a Nd:YVO (4)-KTP green laser. By use of an optoelectronic feedback circuit connected directly to the pump current of the laser diode, the low-frequency intensity noise of the intracavity frequency doubler was suppressed to some extent.

Low-intensity-noise operation of Nd:YVO_4 microchip lasers by pump-noise suppression

We report on near-quantum-limited intensity noise of Nd:YVO4 microchip lasers pumped with pump-noise-suppressed diode lasers. The low-frequency intensity noise of the microchip lasers is found to depend on mode correlation effects in the absorbed radiation from the diode laser pump source. A minimum intensity noise of 0.5 dB above the standard quantum-noise limit at a frequency of 250 kHz is obtained by pumping with a grating-feedback diode laser. An accurate description of the measured intensity-noise spectra by a quantum-mechanical Langevin rate-equation model is achieved by consideration of the nonlinear gain saturation that is due to the finite lifetime of the lower laser level.

Low-frequency intensity noise in semiconductor lasers

The low-frequency intensity noise at 25 MHz of a Fabry-Perot semiconductor laser is measured as a function of injection current. All the measurements are taken at room temperature and the laser is operated with a commercial current source (the conditions under which laser diodes are often used). At the highest injection current of twice threshold, the intensity noise is 5.5 dB above the shot-noise limit. When the longitudinal side mode suppression of the laser is 20 dB or larger, the intensity noise is modeled adequately by an expression derived from the single-mode, small-signal, linearized, semiclassical rate equations. All the parameters used in the theory are derived or referenced.

External phase-modulation interferometry

We analyze and test a laboratory benchtop version of a compound interferometric phase sensor, a Michelson interferometer whose output is combined coherently with a phase-modulated local oscillator beam tapped off the Michelson input beam. This configuration models a whole class of external-modulation interferometers designed to shift signals, obscured by low-frequency intensity noise of the light source, into a shot-noise-limited region of the photocurrent spectrum. We find analytically that the shot-noise-limited sensitivity achievable with this system is comparable with that obtained by using internal phase modulation, with both schemes suffering (for different reasons) approximately a 22% sensitivity penalty compared with ideal shot-noise-limited direct detection. Experimentally we achieve true shot-noise-limited sensitivity, and we investigate trade-offs necessitated by commonly encountered nonideal features in any external-modulation system. Our analytic model, which specifically accounts for Michelson fringe contrast, electronic receiver noise, phase-modulation depth, and the local oscillator tap-off fraction, is sufficiently accurate to predict the absolute sensitivity of our benchtop instrument to within 0.5 dB.

Chaotic dynamics of semiconductor lasers with phase-conjugate feedback.

This paper considers the chaotic dynamics of semiconductor lasers in the presence of phase-conjugate feedback (PCF). Bifurcation diagrams are used to explore the chaotic dynamics and show the difference between the conventional feedback and the PCF. In general, semiconductor lasers display richer chaotic dynamics in the case of PCF. Period-doubling, quasiperiodic, and intermittency routes to chaos are observed in numerical simulations performed by using realistic parameter values. For weak values of PCF, the laser can be phase locked to the phase provided by the phase-conjugate mirror, resulting in dramatic narrowing of the laser linewidth. Higher values of feedback result in periodic output, as the laser relaxation oscillations become destabilized. At a critical value of feedback, the laser enters the chaotic regime, resulting in increased low-frequency intensity noise and substantial broadening of the laser line. Finally, the effect of detuning between the solitary-laser frequency and the frequency of the phase-conjugate-mirror pump laser is explored.

Temperature dependence of mode distribution, intensity noise, and mode-partition noise in subcarrier multiplexed transmission systems

Low-frequency intensity noise generated by Fabry-Perot lasers, which is frequency translated inband by subcarrier modulation and enhanced by dispersion, limits transmission of video signals in subcarrier multiplexed systems. This noise is highly dependent on mode distribution and varies dramatically with temperature. A correlation between low-frequency intensity noise, frequency translated intensity noise, mode-partition noise, and mode distribution is demonstrated. Mode distributions are identified which yield very low or very high levels of noise. Tight control of laser temperature and bias current is required to maintain minimum noise levels in subcarrier transmission systems.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Generation of intensity-correlated twin beams using series-coupled semiconductor lasers.

The low-frequency intensity noise of high-quantum-efficiency lasers or light-emitting diodes (LED's) is dominated by the pump noise. By coupling two or more semiconductor lasers (or LED's) in series with a current source, the pump noise of the two lasers will be equal, and the intensity noise of the lasers will be highly correlated. It is shown that one may generate sub-Poissonian light by measuring the intensity fluctuations of one of the beams and feeding back (or forward) the information to regulate the other photon beam

Demonstration of low-frequency intensity noise reduction for fiber sensors powered by diode-pumped Nd-YAG lasers

Diode-pumped Nd-YAG lasers are being used in fiber sensors for their frequency stability which alleviates the requirements on interferometer path balancing associated with semiconductor lasers. Unfortunately, low-frequency intensity fluctuations induced by the broad-area laser pump, as well as noise associated with launching light into a single-mode fiber, can limit the sensor's minimum detectability. The authors measured the low-frequency relative intensity noise (RIN) of these lasers, implemented feedback into an external modulator to compensate for both laser and launching noise, and reduced the RIN to -100 dB/ square root Hz at 1 Hz. >

Measurements of low frequency and high frequency relative intensity noise in DBR lasers

The relative intensity noise in DBR lasers has been measured as a function of the grating current in both the low frequency and high frequency regimes. With increasing grating current, large increases were only observed in the low frequency intensity noise (&lt;700 MHz). The high frequency intensity noise (&gt;2GHz) was observed to be independent of the grating current.

Low-frequency intensity noise resonance in an external cavity GaAs laser for possible laser characterization

The dependence of the (low-frequency) resonance of light intensity noise in an external-cavity GaAs laser on cavity length and bias current, combined with the dependence of the threshold current on the magnitude of the light feedback, is used to determine simultaneously the carrier recombination lifetime, the carrier injection at transparency, and the optical loss. The key to this technique is understanding the correction term to the relaxation oscillation frequency due to finite carrier density at transparency and the influence of the long external cavity transit time on the photon lifetime in the three-mirror cavity. The technique uses commercial lasers without modifying them and does not require fast electronics.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Analysis of sensitivity and low-frequency intensity noise characteristics of Bragg reflector lasers induced by reflected light

An analysis of the sensitivity and low-frequency intensity noise characteristics of Bragg reflector lasers is given. It is found that Bragg reflector lasers have increased sensitivity to reflected light for low grating coupling strength (кl) and is similar to that of Fabry-Perot lasers except for highКl values. Also, the intensity noise can be reduced by operating the device at high injection level.