Laser Intensity Noise Research Articles

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.

Numerical modeling of intensity and phase noise in semiconductor lasers

A self-consistent numerical approach is demonstrated to analyze intensity and phase noise in semiconductor lasers. The approach takes into account the intrinsic fluctuations of the photon number, carrier number, and phase. A new systematic technique is proposed to generate the Langevin noise sources that derive the laser rate equations keeping their cross-correlations satisfied. The simulation is applied to AlGaAs lasers operating in a single mode. The time-varying profiles of the fluctuating photon and carrier numbers and the instantaneous shift of the oscillating frequency are presented. Statistical analysis of the intensity and phase fluctuations is given. The frequency spectra of intensity and phase noise are calculated with help of the fast Fourier transform. The importance of taking into account the carrier number noise source and its cross-correlation with the noise source on the phase is examined by comparing our results with those by conventional methods.

Characteristics of Intensity Noise Reduction of Optical Pulses Using Variable Spectral Filters and Optical Fibers

Noise reduction of optical pulses using variable spectral filters and optical fibers in the anomalous dispersion region is investigated numerically and experimentally. In the numerical analysis, quantum noise evolution of the optical pulse is calculated using a quantized nonlinear Schrödinger equation and linearization approximation. It is shown that when the optical spectrum is in the broadening process, photon-number squeezing can be obtained using a band-pass filter. On the other hand, when the optical spectrum is in the narrowing process, a correlated pulse pair is generated using a band-rejection filter and photon number squeezing can be also observed. In the experiment, the characteristics of noise reduction of optical pulses are investigated using a passively mode-locked fiber laser at 1.55 µm and a variable spectral filter. Noise distribution of optical pulses changes periodically as the fiber input power is changed. Intensity noise of the fiber laser is reduced by as much as 23 dB using the band-pass filter and is slightly reduced below the shot noise level. Using the band-rejection filter, the noise reduction by the correlated pulse pair is confirmed. The characteristics of the experimental results are almost in agreement with those of the numerical ones.

Microscopic model of quantum noise in single-mode semiconductor lasers

We analyze the zero-frequency intensity noise of a single-mode semiconductor laser on the basis of a full quantum-mechanical model which takes into account the carrier distributions over the k states. The theory includes the effects of (i) Coulomb scattering processes, which tend to drive the carriers into intraband Fermi-Dirac quasiequilibrium; (ii) spectral hole burning, i.e., the carrier deviation from quasiequilibrium induced mainly by light-matter interaction; (iii) pump blocking, which, in conformity with the Pauli exclusion principle, prevents the pump carriers whose k state is already occupied from entering the active layer. Our analysis shows that Coulomb scattering and spectral hole burning have only negligible effects on the noise properties of the laser, while pump blocking can bring, under appropriate parametric conditions, a sizable increase of noise intensity. This additional noise can explain why quietly pumped semiconductor lasers fail to reach the perfect squeezing which is expected on the basis of the theory so far available. The squeezing-hindering effect of pump blocking is confirmed as different pumping schemes are employed.

Passive phase demodulation and laser-noise compensationscheme for fibre interferometers

A scheme for passive phase demodulation and laser intensity noise suppression for fibre interferometers is presented. The scheme uses a matched Ronchi grating filter to transform the interference fringe pattern formed between the reference and the signal fibres into intensity variation that can be related to the optical differential phase in the interferometer. By dividing the grating filter into four sectors with their fringe positions slightly shifted to introduce an equivalent of π/2 phase difference between the signals detected by individual sectors, one can apply cross-subtraction on diagonal sectors to remove intensity noise. In addition, the phase information due to the detected sample vibration can be recovered by taking root-mean-square on the differential signals. The new scheme greatly reduces the complexity of practical fibre interferometers and an experimental setup has been implemented to demonstrate its expected merits.

Self-aligned extended-cavity diode laser stabilized by the Zeeman effect on the cesium D_2 line

An extended-cavity diode laser at 852 nm has been built especially for the purpose of cooling and probing cesium atoms. It is a compact, self-aligned, and continuously tunable laser source having a 100-kHz linewidth and 60-mW output power. The electronic control of the laser frequency by the piezodriven external reflector covers a 4.5-kHz bandwidth, allowing full compensation of acoustic frequency noise without any adverse effect on the laser intensity noise. We locked this laser to Doppler-free resonances on the cesium D(2) line by using the Zeeman modulation technique, resulting in the frequency and the intensity of the laser beam being unmodulated. We also tuned the locked laser frequency over a span of 120 MHz by using the dc Zeeman effect to shift the F = 4-F' = 5 reference transition.

Laser Stabilisation for the Measurement of Thermal Noise

In order to measure the thermal noise of a mirror suspended in a vacuum it is necessary for the length measurement error due to intensity and frequency noise of the probe laser to be reduced below the thermal noise level. Here we report on an experiment to reduce the frequency and intensity noise of a 40mW Nd:YAG laser for this purpose. The frequency is stabilised using the standard reflection locking technique. To stabilise the laser intensity a technique which uses the properties of an ‘in loop’ light field has been developed. This technique is capable of suppressing the intensity noise below the shot noise limit without reducing the useful laser power. A servo based on this technique has been designed and tested. The experimental results indicate that the laser noise can be reduced to a level which will allow a displacement sensitivity of 1.5 × 10-19m/\(\sqrt {{\text{Hz}}}\)Hz for the detection of thermal noise in a frequency band of 10 to 500Hz.

Spatial distribution of the intensity noise of a vertical-cavity surface-emitting semiconductor laser

We studied anticorrelated quantum fluctuations between the TEM(00) and the TEM(01) transverse modes of a vertical-cavity surface-emitting semiconductor laser by measuring the transverse spatial distribution of the laser beam intensity noise. Our experimental results are found to be in good agreement with the predictions of a phenomenological model that accounts for quantum correlations between transverse modes in a light beam.

Effects of pump fluctuations on intensity noise of Nd:YVO microchip lasers

The principle of pump noise suppression is applied to a Nd:YVO4 microchip laser, optically pumped by laser diodes. The noise of the microchip laser at low frequency (below the relaxation oscillation frequency) is compared for noisy and amplitude squeezed laser diodes. The minimum intensity noise of the microchip laser is 7 dB above SNL at a frequency of 40 kHz. Very good agreement between experimental results and theoretical predictions of a model based on quantum Langevin equations is found.

Distributed balanced photodetectors for high-performance RF photonic links

A novel velocity-matched distributed balanced photodetector with a 50-/spl Omega/ coplanar waveguide output transmission line has been experimentally demonstrated in the InP-InGaAs material system. Distributed absorption and velocity matching are employed to achieve high-saturation photocurrent. A common mode rejection ratio of 27 dB has been achieved. The RF link experiment conducted at 6.48 GHz shows that the laser intensity noise has been suppressed by more than 17 dB.

Quantum-mechanical interference between optical transitions: The effect of laser intensity noise

In a previous publication [Phys. Rev. A 55, 552 (1997)] we considered the effect of laser phase fluctuations on three-photon--one-photon phase control of resonance-enhanced photoionization. Here we extend those studies by considering the role of laser intensity noise in addition to laser phase noise. While our results indicate that relative intensity fluctuations between the fundamental field and its third harmonic have a significant effect on control, the contrast between constructive and destructive interference is nonetheless two orders of magnitude, even under (reasonable) worst case situations. Consequently, neither laser intensity nor phase fluctuations appear to pose a serious impediment to the efficient phase control of atomic and molecular processes.

Dynamic noise responses of DFB fibre lasers inpresence of pump power fluctuations

A model is presented for relative intensity noise (RIN) in DFB fibre lasers which predicts the measured characteristics accurately. Calculation results imply that the RIN decreases rapidly with a stronger Bragg grating and higher pump power.

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.

Understanding and controlling laser intensity noise

We present unified theoretical expressions for laser intensity noise in the presence of injection locking and feedback. We discuss optimum control strategies for different configurations and frequency regions. We illustrate the various effects with experimental results from Nd : YAG non-planar ring oscillator lasers.

Influence of nonlinear gain and loss on the intensity noise of a multimode semiconductor laser

Influence of nonlinear gain and loss on the intensity noise of a multimode semiconductor laser

Output power saturation characteristics of the CW-operated semiconductor Raman laser

The intensity noise of a CW-operated semiconductor Raman laser with a tapered waveguide structure with GaP core and AlxGa1-xP cladding layers has been measured and it has been found that the noise spectral intensity in the frequency range 10 Hz–100 kHz was reduced to 34 dB below that of the incident pump light under saturation conditions. This effect has been interpreted as coupling between the pump light, the first Stokes light and the second Stokes light, assuming a very small contribution of nonlinear absorption.

Conversion of laser phase noise to amplitude noise in an optically thick vapor

As laser light propagates through a resonant vapor, laser phase noise (PM) is converted to laser intensity noise (AM) because of the sensitivity of atomic coherence to laser phase fluctuations. In experiments reported here it is shown that this PM-to-AM conversion process is highly efficient and can cause the relative intensity noise of transmitted diode laser light to be 1 to 2 orders of magnitude larger than the laser’s intrinsic relative intensity noise. By use of a semiclassical description of the phenomenon, including the effect of optical pumping, reasonably good agreement between theory and experiment is obtained. The PM-to-AM conversion process discussed here has important consequences for atomic clock development, in which diode-laser optical pumping in thick alkali vapors holds the promise for orders-of-magnitude improvement in atomic clock performance.

Feedback-induced chaos and intensity-noise enhancement in vertical-cavity surface-emitting lasers

We investigate numerically the effects of optical feedback on the intensity noise of vertical-cavity surface-emitting lasers (VCSEL’s) under single-mode and two-mode operations. Our model includes transverse effects such as carrier diffusion and spatial hole-burning. Results indicate that the relative-intensity noise is relatively unaffected at low feedback levels except for a narrowing and enhancement of the relaxation-oscillation peak. At higher feedback levels, the VCSEL ceases to operate continuously, and its output becomes chaotic, following a period-doubling or a quasi-periodic route depending on the feedback level and the length of the external cavity. In the chaotic regime, the relative-intensity noise is enhanced by more than 20 dB. The critical feedback level at which the VCSEL output becomes chaotic depends on whether the laser operates in one or two transverse modes and how strongly these transverse modes are coupled through spatial hole-burning.

Feedback control of laser intensity noise

A fully quantum-mechanical model of feedback to the pump source of a four-level laser is developed with a view to predicting the achievable intensity noise reduction. For solid-state lasers, the model shows that quantum noise sources due to laser dynamics have a significant impact on the optimization of the feedback loop. Experimental results obtained with a diode pumped neodymium:yttrium aluminum garnet (Nd:YAG) laser are found to be in good agreement with the theoretical model. The ultimate limit to the noise suppression comes from the quantum noise due to the measurement processes in the feedback loop. A scheme to overcome this limit using a squeezed vacuum is theoretically demonstrated to be a highly efficient method of generating bright intensity squeezed light, particularly in the kHz regime where bright squeezing is otherwise difficult to obtain.

Feedback control of the intensity noise of injection locked lasers

We demonstrate close to optimum control of the intensity noise of a laser by combining the techniques of laser injection locking and negative electronic feedback. The greatly increased stability of the electronic feedback loop from the output of the slave laser to its pump source that can be obtained by injection locking the slave laser is used to strongly suppress the intensity noise of this laser. We simultaneously use the injection locked output to negatively feed back to an amplitude modulator in the master laser beam and, with both control loops running, we can demonstrate intensity noise suppression of the composite system to within 1 dB of the theoretical limit at 5 kHz.