Quantum noise of laser diodes

Abstract

Single-mode laser theory for semiconductor lasers predicts sub-Poissonian light generation for a laser quietly driven far above threshold. Experiments have shown however that only few laser diodes exhibits such reduced intensity noise. We present a review of different mechanisms that have been proposed to explain the excess noise observed in semiconductor lasers, including imperfect anticorrelations in multimode lasers, and Petermann excess noise factor in single-mode lasers.     

Two-wavelength phase-shifting interferometry insensitive to the intensity modulation of dual laser diodes

We have constructed two-wavelength phase-shifting interferometry that is insensitive to the intensity changes in interferograms associated with the current variations in two laser-diode (LD) sources by using a newly developed phase-extraction algorithm. The tested phase at a synthetic wavelength can be measured from six interferograms with different phase shifts. The algorithm becomes a simple form for seven interferograms and reduces to a minimum of five phase-shifted data in the proper conditions. We shifted the phases equally in opposite directions to one another by separately varying the stepwise currents in dual LD's on an unbalanced interferometer. The measurement accuracy has been improved compared with that of the two-wavelength four-step method. The phase error caused by the power changes in the dual LD's has been investigated theoretically and experimentally. The experimental results are shown to measure a step object with a 4.6-μm synthetic wavelength.     

Multiple-wavelength digital holographic interferometry using tunable laser diodes

Here we present multiple-wavelength digital holographic interferometry with a wide measurement range using laser diodes. Small wavelength differences can be easily realized by the wavelength tuning of laser diodes with injection current controls. A contour map of an object with a wide measurement range and a high sensitivity is demonstrated by combining a few contour maps with several measurement sensitivities. Synthetic wavelengths are calibrated using a known height difference. This alleviates the need to have high precise knowledge of the recording wavelengths. The synthetic wavelengths ranged from ~3 mm for high measurement sensitivity to ~4 cm for wide measurement range. An rms error of ~35 mum for a ~1 cm height measurement is shown. The measured profile of holographic interferometry agrees with a standard stylus instrument.     

On the flicker noise of ferrite circulators for ultra-stable oscillators

The flicker noise of the ferrite circulator is a critical element in ultra-stable microwave oscillators, in which the signal reflected from the input of the reference cavity is exploited to stabilize the frequency. This paper explains why the circulator noise must be measured in isolation mode, proposes a measurement scheme, and provides experimental results. The observed flicker spans from -162 to -170 dB[rad2]/Hz at 1 Hz off the 9.2 GHz carrier, and at +19 dBm of input power. In the same conditions, the instrument limit is below -180 dB[rad2]/Hz. Experiments also give information on the mechanical stability of the microwave assembly, which is in the range of 10(-11) m. The measurement method can be used as the phase detector of a corrected oscillator; and, in the field of solid-state physics, it can be used for the measurement of random fluctuations in magnetic materials.     

Limitations of silicon diodes for clinical electron dosimetry

This work investigates the relevance of several factors affecting the response of silicon diode dosemeters in depth-dose scans of electron beams. These factors are electron energy, instantaneous dose rate, dose per pulse, photon/electron dose ratio and electron scattering angle (directional response). Data from the literature and our own experiments indicate that the impact of these factors may be up to +/-15%. Thus, the different factors would have to cancel out perfectly at all depths in order to produce true depth-dose curves. There are reports of good agreement between depth-doses measured with diodes and ionisation chambers. However, our measurements with a Scantronix electron field detector (EFD) diode and with a plane-parallel ionisation chamber show discrepancies both in the build-up and in the low-dose regions, with a ratio up to 1.4. Moreover, the absolute sensitivity of two diodes of the same EFD model was found to differ by a factor of 3, and this ratio was not constant but changed with depth between 5 and 15% in the low-dose regions of some clinical electron beams. Owing to these inhomogeneities among diodes even of the same model, corrections for each factor would have to be diode-specific and beam-specific. All these corrections would have to be determined using parallel plane chambers, as recommended by AAPM TG-25, which would be unrealistic in clinical practice. Our conclusion is that in general diodes are not reliable in the measurement of depth-dose curves of clinical electron beams.     

Limitations to the omvpe growth of Hg compounds due to hydrodynamic effects

The use of organometallic vapor phase epitaxy as a growth technique for the growth of Hg-containing semiconductors has been hindered due to lack of reproducibility and uniformity. In this work, the role of transport processes in the vapor phase growth is considered. Specifically we use numerical simulation to investigate two effects that are likely to adversely influence the growth uniformity and reproducibility. First, the growth rate can be affected by relatively small temperature gradients on the reactor walls, since these affect the Hg vapor concentration in the neighborhood of the growing layer. Second, recirculation cells driven by density gradients resulting from Hg depletion near the growing film influence not only the growth rate but also the uniformity of the deposition. This is the first case where solutal convection, well known in bulk crystal growth, has been predicted to have a significant impact on OMVPE process performance.     

Susceptibility of systematic error-compensating algorithms to random noise in phase-shifting interferometry

In phase-shifting interferometry, many algorithms have been reported that suppress systematic errors caused by, e.g., nonlinear motion of the phase shifter and nonsinusoidal signal waveform. However, when a phase-shifting algorithm is designed to compensate for the systematic phase-shift errors, it becomes more susceptible to random noise and gives larger random errors in the measured phase. The susceptibility of phase-shifting algorithms to random noise is analyzed with respect to their immunity to phase-shift errors and harmonic components of the signal. It is shown that for the most common group of error-compensating algorithms for nonlinear phase shift, both random errors and the effect of high-order harmonic components of the signal cannot be minimized simultaneously. It is also shown that if an algorithm is designed to have extended immunity to nonlinear phase shift, simultaneous minimization becomes possible.     

Crystalline-silicon-based infra-red LEDs and routes to laser diodes

We review progress in silicon LEDs using dislocation engineering to achieve high temperature operation, a process that is fully CMOS (Complementary Metal Oxide Semiconductor) compatible. We concentrate on devices operating in the near infra-red where high value applications are. The need for silicon emitters, lasers and optical amplifiers is discussed followed by an outline of previous approaches and possible future routes explored. Results on gain in silicon are reported and routes to electrically pumped injection lasers and optical amplifiers considered. Extension of 1.1 and 1.5 μm devices to other wavelengths is discussed.     

Microwave interferometry: application to precision measurements and noise reduction techniques

A concept of interferometric measurements has been applied to the development of ultra-sensitive microwave noise measurement systems. These systems are capable of reaching a noise performance limited only by the thermal fluctuations in their lossy components. The noise floor of a real time microwave measurement system has been measured to be equal to -193 dBc/Hz at Fourier frequencies above 1 kHz. This performance is 40 dB better than that of conventional systems and has allowed the first experimental evidence of the intrinsic phase fluctuations in microwave isolators and circulators. Microwave frequency discriminators with interferometric signal processing have proved to be extremely effective for measuring and cancelling the phase noise in oscillators. This technique has allowed the design of X-band microwave oscillators with a phase noise spectral density of order -150 dBc/Hz at 1 kHz Fourier frequency, without the use of cryogenics. Another possible application of the interferometric noise measurements systems include “flicker noise-free” microwave amplifiers and advanced two oscillator noise measurement systems     

Correction to the noise temperature of a waveguide thermal noise generator due to nonuniform heating of the channel

The correction to the noise temperature of waveguide thermal noise generators due to nonuniformity of the heating of the waveguide line is considered. It is shown that, for a simplified model of a thermal noise generator, the minimum values of the corrections in a copper waveguide with dimensions of 2.4×1.2 mm and a waveguide of stainless steel with dimensions of 3.6×1.8 mm amount to 0.05 K and 0.15 K respectively, for a length of the nonuniformly heated part of the waveguide line of 80 mm. Translated from Izmeritel'naya Tekhnika No. 5, pp. 46–49, May, 1998.     

Measurement of liquid-film thickness by laser interferometry

Here the variation of a liquid-film thickness at small Reynolds numbers is discussed. The film thickness measurement by laser interferometry corresponds to the liquid flowing on the inner surface of a small-bore glass tube. An adequate theoretical background for the techniques used in this experiment is discussed to demonstrate the capability of the experimental technique. An advantage of this method is that it shows the shape of the thin film on the inner surface of the vertical tube at a point in a horizontal cross section. The results obtained from this experiment show that the flow of liquid films on vertical surfaces is inherently unstable and three dimensional even at a Reynolds number smaller than 1.     

Measurement of in plane residual displacement due to interference fitting by TV-holographic interferometry

Experimental measurements of the in plane residual displacement field surrounding the cold expanded holes in aluminium plates of 100mm × 100mm × 10mm were made using the technique of TV-holographic interferometry. A special cold expansion fixture was designed to produce precise radial interferences of 0.015mm and 0.025mm at the hole walls. The experimentally determined displacement fields were compared with those obtained by finite element predictions. The experimental results showed good agreement with those obtained by finite element analysis. These results demonstrate that the TV-holographic interferometry technique can be used as an effective technique for general deformation investigations.     

The state of the art of flicker frequency noise in BAW and SAW quartz resonators

Experimental results of the last 15 years are reviewed. Noise properties of crystal filters and oscillators are reported, along with practical measurements. It is shown that the additional phase fluctuations are compensated by frequency fluctuations and vice versa. With the assistance of these theoretical results the flicker and white frequency noise coefficients, h(-1) and h(0), respectively, are plotted versus unloaded Q and carrier frequency f(0) for the measured and published crystal oscillator noise characteristics. The dependence of h(-1) approximately 10(-12.75) Q(2) (u) is verified.     

车轮扁疤激起的轮轨冲击噪声机理分析

摘　要 为了预测和控制快速和准高速线路上车轮扁疤激起的轮轨冲击噪声,应用车辆－轨道耦合动力学理论和声辐射理论,建立了基于车辆－轨道相互作用的轮轨冲击噪声预测模型,编制轮轨噪声仿真软件TTINSIM计算分析了车轮扁疤激扰下轮轨冲击噪声的特性。结果表明：（1）车轮扁疤是造成轮轨冲击噪声的重要源泉;（2）车轮扁疤引起的冲击噪声主要集中在250Hz以上频段;（3）车轮扁疤引起的轮轨冲击噪声跟扁疤长度、扁疤个数以及列车运行速度有很大的关系;（4）车轮新扁疤比旧扁疤激起的轮轨冲击噪声大2～3dB(A)。     

Spectral noise due to sampling errors in Fourier-transform spectroscopy

An assessment is made of the spectral noise in Fourier-transform spectroscopy caused by sampling errors in the interferogram acquisition. Numerical evaluations are performed in the case of the REFIR (radiation explorer in the far infrared) instrument developed for the measurement of the long-wavelength Earth emissions from satellite platforms. In this case the slow response of a room-temperature pyroelectric detector, the relatively short acquisition time, the broadband operation, and the wish for a relaxed requirement of the mirror drive accuracy make sampling error an important issue. Different sampling methods can be considered for reduction of the spectral noise induced by sampling errors. The effects of different sampling methods are quantified and discussed for the selection of the most-suitable option for this instrument. We find that only sampling methods that introduce some compensation (either analog or digital) of the frequency dependence of amplitude and phase components of the acquisition-system responsivity provide satisfactory results. In particular, the equal time sampling followed by a digital filter and numerical resampling has been examined minutely with a simulation model used to perform sensitivity tests of the main parameters that characterize the procedure.