Quantum Noise Limit Research Articles

Analysis of noise figure spectral distribution in erbium doped fiber amplifiers pumped near 980 and 1480 nm

A small signal analysis of the noise figure spectral distribution in erbium doped fiber amplifiers pumped near 980 and 1480 nm is presented. In the case where signal-spontaneous beat noise is the dominant cause of signal-to-noise ratio (SNR) degradation, it is shown that noise figures in the +/-0.2-dB range around the 3-dB quantum limit are possible within a spectral band of 50 nm, with the result applying to lambda(p) = 980-nm and lambda(p) = 1460-nm pump wavelengths. For lambda(p) > 1460 nm, a noise figure penalty of 0.5 to 1.2 dB above the quantum limit is found. This study thus demonstrates the possibility of quantum limited or near quantum limited noise regime for amplification of wavelength division multiplexed signals in erbium doped fibers.

Heterodyne detection with an injection laser. II. Signal-to-noise ratio

For pt.I see ibid., vol.26, p.85-93 (1990). In part I a theory of the conversion efficiency of the self-heterodyne laser detector that was first described by R.F. Kazarinov and R.A. Suris (1974) was presented. In this device a light signal is passed into the resonant cavity of an actively oscillating injection laser, causing an electrical signal at the difference frequency between laser and signal to flow through the wire supplying the DC bias to the laser. An expression is derived for the signal-to-noise ratio of the self-heterodyne laser detector. The result shows that in the limit of ideal operation (complete population inversion and no internal losses) the signal-to-noise ratio of the self-heterodyne laser detector reaches one-half of the quantum noise limit. >

Two-tone frequency-modulation spectroscopy with a CO_2 laser

We report the laboratory demonstration of two-tone frequency-modulation spectroscopy with a continuous-wave (cw) CO2 laser. Using a resonant-cavity electro-optic phase modulator operating at 1 GHz, we obtained sensitivities to absorptions of 2 × 10−5 in a 1-Hz bandwidth from an étalon resonance and from calibrated mixtures of NH3. This sensitivity was within a factor of 3 of the quantum noise limit for our apparatus and was limited by baseline drift and fluctuations due to residual amplitude modulation and optical fringes.

Excess spontaneous emission in non-Hermitian optical systems. I. Laser amplifiers.

Petermann first predicted in 1979 the existence of an excess-spontaneous-emission factor in gain-guided semiconductor lasers. We show that an excess spontaneous emission of this type, and also a correlation between the spontaneous emission into different cavity modes, will in fact be present in all open-sided laser resonators or optical lens guides. These properties arise from the non-self-adjoint or non-power-orthogonal nature of the optical resonator modes. The spontaneous-emission rate is only slightly enhanced in stable-resonator or index-guided structures, but can become very much larger than normal in gain-guided or geometrically unstable structures. Optical resonators or lens guides that have an excess noise emission necessarily also exhibit an ``excess initial-mode excitation factor'' for externally injected signals. As a result, the excess spontaneous emission can be balanced out and the usual quantum-noise limit recovered in laser amplifiers and in injection-seeded laser oscillators, but not in free-running laser oscillators.

Laser heterodyne spectrometer for monitoring atmospheric minor constituents.

An infrared laser heterodyne spectrometer has been developed to measure solar absorption lines of atmospheric minor constituents by using a PbSnTe diode laser and a CO2 laser as local oscillators. The system is capable of operating aboard an airplane in order to detect stratospheric molecules in very low concentrations by observing the sun near sunset or sunrise at the horizontal direction. The test operation on the ground shows that the sensitivity of the photomixer attains the quantum noise limit when a diode laser as well as a CO2 laser is used and the number of absorption lines of stratospheric ozone are successfully measured.

Realization of ultrahigh coherence in semiconductor lasers by negative electrical feedback

To be useful, an ultrahigh-coherence semiconductor laser source requires high frequency stability, narrow linewidth, the capability of frequency tracking to a master laser, and stable frequency tuning. Negative electrical feedback is proposed to meet these four requirements simultaneously. Although the degree of frequency fluctuation that can be reduced by negative electrical feedback is limited by the noise contained in the feedback signal, theoretical calculations show that the fluctuations can be lower than the quantum noise limit. Experimental results obtained recently by the author are reviewed. >

Squeezed Quantum Fluctuations and Noise Limits in Amplifiers and Attenuators

Squeezed Quantum Fluctuations and Noise Limits in Amplifiers and Attenuators

Multiport homodyne detection near the quantum noise limit

A detailed description of a multiport detection experiment operating close to the quantum noise limit is presented. Simultaneous phase and amplitude measurements were made on signals of only five photons energy with a signal-to-noise ratio that was accountable to within 2 dB of the quantum limit. Statistical examination of the noise showed excellent agreement with the gaussian distribution predicted by both the semiclassical and fully quantum theories.

Ultrasensitive detection of light absorption by few ions

Small clouds of Ba ions confined in a miniature rf trap have been probed with Doppler-free cross-modulating sideband absorption of FM laser light. The quantum noise limit of the heterodyne four-wave mixing signal corresponds to 65 detected ions, as determined from single-ion fluorescence. This is so far the most sensitive detection of the absorption of a light beam.

Efficient capacity for a PPM weak noisy photon-counting channel

Results on the efficient capacity of a noisy PPM photon-counting channel are presented. Optimization of the optical system over the number of slots, the average power of the signal and the frequency bandwidth of the channel is considered and studied. The regions of quantum-limited operations and weak background noise are analysed in detail.

Potential for coherent Doppler wind velocity lidar using neodymium lasers

Existing techniques for the frequency stabilization of Nd: YAG lasers operating at 1.06 μm, and the high-gain amplification of radiation at that wavelength, make possible the construction of a coherent Doppler wind velocity lidar using Nd:YAG. Velocity accuracy and range resolution are better at 1.06 μm than at 10.6 μm at the same level of the SNR. Backscatter from the atmosphere at 1.06 μm is greater than that at 10.6 μm by ~2 orders of magnitude, but the quantum-limited noise is higher by 100 also. Near-field attenuation and turbulent effects are more severe at 1.06 μm. In some configurations and environments, the 1.06-μm wavelength may be the better choice, and there may be technological advantages favoring the use of solid-state lasers in satellite systems.

Effectiveness of antiscatter grids in digital subtraction angiography.

An analysis of the potential value of antiscatter grids for digital subtraction angiography (DSA) is presented. Using the Rose criterion for signal detection, and assuming quantum limited statistical noise, it is shown that grid selectivity and primary transmission are the two most important parameters of grid performance for DSA. In comparison with conventional grids commonly in current use in radiology departments, a grid having a higher level of primary transmission, (approximately 90%) and a moderate level of selectivity (approximately 6) could provide comparable imaging signal-to-noise ratios, but with 20% to 45% smaller patient dose. Implications of these results are discussed.

Analysis of strong inductive coupling on SQUID systems

A technique for the analysis of complex superconducting circuits containing one or more current biased inductively coupled Josephson junction interferometers is described. The equations of motion for the system are shown to separate into a set of non-linear junction equations relating the voltage drops across the junctions to the currents passing through the junctions, and a set of linear equations expressing the quantization of the fluxoid around the superconducting loops and the corresponding linear circuit equations for the normal loops. In many cases, the linear equations can be integrated, resulting in an effective elimination of the normal circuit loops from the problem. The circuit is thus reduced to that of an equivalent set of coupled SQUIDs whose device parameters are a function of the original circuit parameters and the inductive coupling strengths. The dc SQUID tuned linear amplifier is analyzed to display the method. As a result of this analysis, the optimal noise temperature of a tuned SQUID amplifier is derived as a function of the current and voltage noise spectral densities, S v and S j , and forward transfer function, V φ . The results obtained demonstrate that the behavior of the SQUID cannot be isolated from that of the rest of the circuit in the manner assumed in previous analyses. The existence of a quantum noise limit on the noise temperature of any phase preserving linear amplifier is used to infer that the noise figures must satisfy the constraint [(S_{v}/2LV_{\phi}^{2})(LS_{j}/2)-S_{vj}^{2}]^{1/2}>\hbar The behavior of several simple circuits containing a pair of inductively coupled current biased interferometers is discussed to display the extension of the method to circuits containing more than one interferometer.

Book reviews

The book is ideal for engineers, scientists and senior students who are interested in optical fiber transmission systems and the physical mechanisms that influence their design. Basic explanations of component characteristics, such as LED's, laser diodes and optical fibers are given. Compromises that must be made when designing an optical transmission system are explained clearly. The quantum noise limit of signal detection (−77 dBm), for example, is derived for a 10M baud/s system with a 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−9</sup> error rate. This is later compared to a practical receiver design that can operate efficiently with an input of −64 dBm.

Optimization of dc SQUID linear amplifiers and the quantum noise limit

A model for inductively coupled linear amplifier and magnetometer circuits employing superconducting quantum interference devices (dc SQUID’s), which is accurate to all values in the coupling strength, is described. The model explicitly includes the interaction between the input circuit and the dc SQUID. The optimization of the untuned linear amplifier is discussed in detail. The uncertainty principle constraint on the noise temperature of a linear amplifier is shown to place a constraint on the minimum achievable values of the noise figures of an isolated dc SQUID in the quantum noise limit.

Time-resolved inverse Raman spectroscopy

A technique for obtaining sensitive, highly reproducible, time-resolved inverse Raman measurements is reported. Experimental results are presented for the nitrogen vibrational Q branch at a pressure of 10 atm. For these measurements the signal, normalized to the pump-laser energy, exhibits fluctuations of 1.4% (rms) about the average of 500 measurements; these deviations are within a factor of 2 of the quantum noise limit.

Detection of objects in quantum-noise-limited images

A method is presented that allows the automatic detection of single particles against an extremely noisy background, e.g., biological macromolecules in a low dose electron micrograph. In this method each image element is replaced by the image variance in its environment. This convolution-like operation leads to the “variance image” which is an efficient detector of objects with the same average density as the background (such as phase objects). The variance image is essentially a simple measure of texture. The human visual system appears to be equipped with an equally powerful detector. A separate class of objects is comprised of objects that have an average density different from that of the background. These objects are often best detected by a low-pass filtering operation. The quantum-noise limits of both methods of detection are calculated and compared to the noise limits of “matched filtering” methods. Fast algorithms are provided to calculate the variance image. Model calculations confirm the theoretical noise limits. The Rose equation is discussed in the context of the two operators treated in this paper.

Dc SQUID near the quantum noise limit

A series of dc SQUIDs has been fabricated in an attempt to obtain quantum-limited sensitivity. Typically, the SQUID inductance, L, is 2 pH, the capacitance and critical current of each tunnel junction are 0.5 pF and 0.5 mA, and the shunt resistance for each junction is 1Ω. The measured spectral density of the voltage noise S v contains a 1/f component that extends typically to 100 kHz or higher. When the 1/f component is subtracted out, the best energy sensitivity achieved to date is ε 1 Hz = S v 2 L ( ∂ V ∂Φ ) 2 2 ̌ h ̵ , where ∂V/∂Φ is the transfer coefficient.

Superconductive devices for millimeter wave detection, mixing, and amplification

Single-particle (quasi-particle) tunneling through an insulating barrier between two superconductors or between a superconductor and a normal conductor is being used to make very-low-noise detectors and mixers for millimeter wavelengths. The nonlinearity of the I-V curve obtained from tunneling between two superconductors can be so strong that classical theory breaks down and photon-assisted tunneling theory must be used to understand device performance. Quantum theory predicts that a quasi-particle tunnel junction can be operated as a microwave photon detector with quantum efficiency close to unity or as a heterodyne mixer with conversion gain and with mixer noise temperature comparable with the quantum noise limit T_{M} =\bar{h}\omega/k . Both of these predictions have been experimentally realized at 36 GHz using superconductor-insulator-superconductor (SIS) junctions. It appears probable that these quasi-particle detectors and mixers will supercede the corresponding Josephson-effect devices at millimeter wavelengths.

Quantum limited detection in tunnel junction mixers

A complete quantum generalization of microwave mixer theory is constructed for nonlinear single-particle tunnel junctions. The result represents a unification of the concepts used to describe these classical resistive mixers with the language of photon detection. Tunneling devices are predicted to undergo a transition from energy detectors to photon counters when operated at frequencies where the photon energy becomes comparable to the voltage scale of the dc nonlinearity. The small-signal video current response is found to approach one electron for each photon absorbed at high frequencies. In a heterodyne receiver, sufficiently nonlinear tunnel junctions are predicted to be capable of achieving the fundamental quantum noise limit for sensitivity in the detection of electromagnetic radiation. The theory presented here thus provides a framework for systematically extending the techniques of quantum electronics to considerably lower frequencies than are currently being exploited. Recent measurements of heterodyne mixer performance using superconductive tunneling devices are already beginning to approach quantum limited results at microwave and millimeter wave frequencies. Eventual application of tunnel barriers as photon detectors in the submillimeter and infrared spectral regions also appears to be possible, and the fast response times of such devices could give them an advantage over photoconductors even at the higher frequencies. The development of suitable nonlinear tunnel junctions contains the potential to bridge the present gap in quantum detectors between the infrared photon devices and microwave masers.