Shot Noise Limited Operation Research Articles

Stabilized laser system at 1550 nm wavelength for future gravitational-wave detectors

Proposed future gravitational wave detectors place high demands on their prestabilized laser system. We present a prototype for such a prestabilized laser system at $1550\text{ }\text{ }\mathrm{nm}$ wavelength with frequency and power stabilizations optimized for the needs of gravitational wave detectors. A power stabilization with shot noise limited operation below a relative power noise of $1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}\text{ }\text{ }{\mathrm{Hz}}^{\ensuremath{-}1/2}$ between 100 Hz to 100 kHz and an active frequency stabilization with a unity-gain bandwidth above 2 MHz were operated simultaneously. Out-of-loop measurements are performed to characterize the achieved stability and to analyze sensor noise limits. We find that nonlinear noise couplings at the spatial mode-filter cavity are of high relevance and lead to increased frequency stability requirements above 100 kHz . This prestabilized laser system can serve as the baseline for the Einstein Telescope gravitational wave detector [ET steering committee, Design report Update 2020 for the Einstein Telescope, Technical Report, Einstein gravitational wave Telescope, 2020.] and demonstrates stabilization concepts generally applicable to optical precision experiments.

Electrically pumped shot-noise limited class A VECSEL at telecom wavelength.

Class A shot-noise limited operation is achieved in an electrically pumped vertical external cavity surface emitting laser (VECSEL), opening the way for integration of such peculiar noiseless laser oscillation in applications where low power consumption and footprint are mandatory. The quantum well active medium is grown on an InP substrate to enable laser oscillation at telecom wavelengths. Single frequency class A operation is obtained by proper optimization of the cavity dimensions, ensuring at the same time a sufficiently long and high-finesse cavity without any intracavity filtering components. The laser design constraints due to electrical pumping are discussed as compared to optical pumping. The intensity noise spectrum of this laser is shown to be shot-noise limited, leading to a relative intensity noise of $-160\;{\rm dB/Hz}$ for 3.1 mA detected photocurrent.

A low-noise resonant input transimpedance amplified photodetector

We present the design and characterization of a low-noise, resonant input transimpedance amplified photodetector. The device operates at a resonance frequency of 90 MHz and exhibits an input referred current noise of 1.2 pA/Hz—marginally above the theoretical limit of 1.0 pA/Hz set by the room temperature Johnson noise of the detector’s 16 kΩ transimpedance. As a result, the photodetector allows for shot-noise limited operation for input powers exceeding 14 µW at 461 nm corresponding to a noise equivalent power of 3.5 pW/Hz. The key design feature which enables this performance is a low-noise, common-source junction gate field-effect transistor amplifier at the input which helps to reduce the input referred noise contribution of the following amplification stages.

Continuous Variable Quantum Key Distribution with a Noisy Laser

Existing experimental implementations of continuous-variable quantum key distribution require shot-noise limited operation, achieved with shot-noise limited lasers. However, loosening this requirement on the laser source would allow for cheaper, potentially integrated systems. Here, we implement a theoretically proposed prepare-and-measure continuous-variable protocol and experimentally demonstrate the robustness of it against preparation noise stemming for instance from technical laser noise. Provided that direct reconciliation techniques are used in the post-processing we show that for small distances large amounts of preparation noise can be tolerated in contrast to reverse reconciliation where the key rate quickly drops to zero. Our experiment thereby demonstrates that quantum key distribution with non-shot-noise limited laser diodes might be feasible.

Real-time coherent detection of phase modulated ultrashort pulses after time-to-space conversion and spatial demultiplexing.

Phase modulated sub-picosecond pulses are converted by a time-to-space processor to quasi-monochromatic spatial beams that are spatially demultiplexed and coherently detected in real-time. The time-to-space processor, based on sum-frequency generation (SFG), serves as a serial-to-parallel converter, reducing the temporal bandwidth of the ultrashort pulse to match the bandwidth of optoelectronic receivers. As the SFG process is phase preserving, we demonstrate homodyne coherent detection of phase modulated temporal pulses by mixing the demultiplexed SFG beam with a narrow linewidth local oscillator (LO) resulting in single-shot phase detection of the converted pulses at a balanced detector. Positively and negatively phase-modulated signal pulses are individually detected and LO shot noise limited operation is achieved. This demonstration of real-time demultiplexing followed by single-shot full-field detection of individual pulses, highlights the potential of time-to-space conversion for ultrahigh bit rate optical communications and data processing applications.

Nonlinear intermodulation distortion suppression in coherent analog fiber optic link using electro-optic polymeric dual parallel Mach-Zehnder modulator

A linearized dual parallel Mach-Zehnder modulator (DPMZM) based on electro-optic (EO) polymer was both fabricated, and experimentally used to suppress the third-order intermodulation distortion (IMD3) in a coherent analog fiber optic link. This optical transmitter design was based on a new EO chromophore called B10, which was synthesized for applications dealing with the fiber-optic communication systems. The chromophore was mixed with amorphous polycarbonate (APC) to form the waveguide's core material. The DPMZM was configured with two MZMs, of different lengths in parallel, with unbalanced input and output couplers and a phase shifter in one arm. In this configuration each of the MZMs carried a different optical power, and imposed a different depth of optical modulation. When the two optical beams from the MZMs were combined to generate the transmitted signal it was possible to set the IMD3 produced by each modulator to be equal in amplitude but 180° out of phase from the other. Therefore, the resulting IMD3 of the DPMZM transmitter was effectively canceled out during two-tone experiments. A reduction of the IMD3 below the noise floor was observed while leaving fifth-order distortion (IMD5) as the dominant IMD product. This configuration has the capability of broadband operation and shot-noise limited operation simultaneously.

Empirical electro‐optical and x‐ray performance evaluation of CMOS active pixels sensor for low dose, high resolution x‐ray medical imaging

Monolithic complementary metal oxide semiconductor (CMOS) active pixel sensors with high performance have gained attention in the last few years in many scientific and space applications. In order to evaluate the increasing capabilities of this technology, in particular where low dose high resolution x-ray medical imaging is required, critical electro-optical and physical x-ray performance evaluation was determined. The electro-optical performance includes read noise, full well capacity, interacting quantum efficiency, and pixels cross talk. The x-ray performance, including x-ray sensitivity, modulation transfer function, noise power spectrum, and detection quantum efficiency, has been evaluated in the mammographic energy range. The sensor is a 525 x 525 standard three transistor CMOS active pixel sensor array with more than 75% fill factor and 25 x 25 microm pixel pitch. Reading at 10 f/s, it is found that the sensor has 114 electrons total additive noise, 10(5) electrons full well capacity with shot noise limited operation, and 34% interacting quantum efficiency at 530 nm. Two different structured CsI:Tl phosphors with thickness 95 and 115 microm, respectively, have been optically coupled via a fiber optic plate to the array resulting in two different system configurations. The sensitivity of the two different system configurations was 43 and 47 electrons per x-ray incident on the sensor. The MTF at 10% of the two different system configurations was 9.5 and 9 cycles/mm with detective quantum efficiency of 0.45 and 0.48, respectively, close to zero frequency at approximately 0.44 microC/kg (1.72 mR) detector entrance exposure. The detector was quantum limited at low spatial frequencies and its performance was comparable with high resolution a: Si and charge coupled device based x-ray imagers. The detector also demonstrates almost an order of magnitude lower noise than active matrix flat panel imagers. The results suggest that CMOS active pixel sensors when coupled to structured CsI:Tl can be used for conventional and advanced digital mammography due to their low noise, high resolution performance.

Development of submillimeter-wave camera for Atacama Submillimeter Telescope Experiment

Submillimeter-wave focal plane arrays based on superconducting direct detectors are designed to be used on a new 10-m submillimeter-wave telescope in Chile. Niobium tunnel junctions with low leakage current coupled to antenna structures can be sensitive submillimeter-wave direct detectors when high quantum efficiency and low leakage current are realized. Inhomogeneous distributed junctions coupled to log-periodic antennas can be used to realize this. Input coupling of better than 50% is calculated with a bandwidth of more than 60 GHz using low current density inhomogeneous distributed junctions. Their measured leakage current of 5 pA and their shot-noise limited operation suggests that noise equivalent power of less than 10/sup -17/W/Hz/sup 0.5/ at operating temperature of 0.9 K could be achieved.

Coherent Optical Communication Systems

This paper presents an overview of coherent optical fiber communication systems. The fundamental principle is described and the performance limits under shot noise limited operation for various modulation and demodulation schemes are compared with respect to the quantum limit. The practical constraints in system implementation and their remedies are outlined, and finally the potential system applications are discussed.

Four-Wave-Mixing in Optical ASK WDM Transmission System Employing Heterodyne Detection

We analyze the impact of four-wave-mixing (FWM) in optical fibers on wavelength-division-multiplexed (WDM) optical communication systems employing amplitude-shift-keying (ASK) and heterodyne detection. The manifestation of fiber FWM in the receiver is viewed as an interference process, and a suitable statistical model is developed for the same including the influence of receiver bandpass filter on the FWM interference. The receiver bit-error-probability (BEP) is evaluated in presence of the FWM interference and the receiver shot noise. For a given fiber, the plot of receiver BEP versus received optical power shows an irreducible minimum which depends on the transmission distance (TD). For a 16 channel 1 Gbit/s WDM ASK system, the maximum TD for a BEP min is 229 km for nondispersion-shifted (NDS) fibers, which is 21 km larger than the same for dispersion-shifted (DS) fibers. The receiver power penalty (RPP) due to the FWM as compared to the shot noise limited operation is also evaluated. For the present system the FWM leads to the RPP of 1.54 dB and 1.39 dB for DS and NDS fibers respectively when operating at the corresponding maximum TDs.

Charge Injection Device (CID) technology — A solution for photon and particle imaging applications

Charge Injection Device (CID) imagers provide key electro-optical performance features and versatile readout capabilities for spatial and radiometric measurements of photon and particle radiation. CIDs have many of the performance features required for machine vision and scientific applications including good resolution, broad spectral response, resistance to blooming and asynchronous operation for feature detection and measurement. Single pixel imaging devices and two-dimensional arrays possess exceptionally large pixel charge capacities and wide dynamic range that provide quantum (shot-noise) limited operation. Commercial devices have demonstrated the robustness and capability to operate in hostile (including ionizing radiation) environments. Recently-developed CID imagers and camera systems have incorporated new on-chip architectures and process technology that provide low-noise performance and permit exploitation of additional features including random-pixel addressability, true non-destructive pixel readout (NDRO), and two-dimensional windowing (subarray readout). These CID features allow scientists to monitor and adapt application exposure in real time, reduce noise, and read out small pixel subarrays at fast rates. CID imagers possess structural characteristics and improved radiation tolerance (∼ 10 6 gamma) which allow these devices to operate in harsh radioactive environments and image different forms of radiation including X-rays, electron beams and nuclear radiation.

High sensitivity and submillimeter resolution optical time-domain reflectometry based on low-coherence interference

Characteristics of the low-coherence optical time-domain reflectometer (OTDR) are presented, and optical waveguide diagnosis using the OTDR is demonstrated. Ultrahigh sensitivity in the shot noise limited operation is discussed, and jagged-shaped Rayleigh-backscatter signals are predicted and observed experimentally. Signal smoothing by widening the resolution from 14 to 400 mu m can drastically reduce the jagging. In the experiment, polarization-sensitive and insensitive OTDRs using a fiber-coiled piezoelectric phase modulator are proposed. Measurements of beat length and bending loss, quality characterization of low-loss waveguides by polarization-sensitive OTDR, and cancellations of polarization fading in discrete backreflection and polarization jagging in Rayleigh backscattering by polarization-insensitive OTDR are successfully demonstrated. >

A comparison of sensitivity degradations for optical homodyne versus direct detection of on-off keyed signals

The sensitivity of an optical homodyne receiver with practical degradations is analytically evaluated using a semiclassical photodetection model which is valid for arbitrarily low levels of local oscillator (LO) power. This model provides several important advantages over the usual treatment in which large LO is assumed. In addition to giving an intuitive picture of the evolution of the detection sensitivity from the direct detection case (LO=0) to the shot noise limited coherent detection case (large LO), it allows for a quantitative comparison between degradations caused by some important receiver nonidealities for the two limiting cases. Sensitivity loss due to dark current, thermal noise, and electronic distortion is evaluated and the LO power required to reach shot noise limited operation in the presence of these degradations is determined.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Optical detection of charge modulation in silicon integrated circuits using a multimode laser-diode probe

This paper reports on the detection of sheet charge densities in silicon devices using an improved noninvasive optical probe based on the detection of free-carrier optical dispersion using a multilongitudinal-mode 1.3-µm semiconductor laser. The improved system incorporates a differential detection technique and a Wollaston prism that allows the use of the multimode laser. These changes increase stability, sensitivity, and bandwidth, allow near shot-noise limited operation, reduce required optical power, and simplify the apparatus. The technique can be applied to probe electronic signals or, conversely, to modulate light using controlled electronic signals. Simple demonstrations of each application are presented.

Optimization of Optical Systems

The power signal-to-noise ratios for coherent and noncoherent optical detection are presented, with the expression for noncoherent detection being examined in detail. It is found that for the long range optical system to compete with its microwave counterpart it is necessary to optimize the optical system. The optical system may be optimized by using coherent detection, or noncoherent detection if the signal is the dominate noise factor. A design procedure is presented which, in principle, always allows one to obtain signal shot-noise limited operation with noncoherent detection if pulsed operation is used. The technique should make reasonable extremely long range, high data rate systems of relatively simple design.