Balanced Detection Technique Research Articles

Fourier domain optical coherence tomography system with balance detection

A Fourier domain optical coherence tomography system with two spectrometers in balance detection is assembled using each an InGaAs linear camera. Conditions and adjustments of spectrometer parameters are presented to ensure anti-phase channeled spectrum modulation across the two cameras for a majority of wavelengths within the optical source spectrum. By blocking the signal to one of the spectrometers, the setup was used to compare the conditions of operation of a single camera with that of a balanced configuration. Using multiple layer samples, balanced detection technique is compared with techniques applied to conventional single camera setups, based on sequential deduction of averaged spectra collected with different on/off settings for the sample or reference beams. In terms of reducing the autocorrelation terms and fixed pattern noise, it is concluded that balance detection performs better than single camera techniques, is more tolerant to movement, exhibits longer term stability and can operate dynamically in real time. The cameras used exhibit larger saturation power than the power threshold where excess photon noise exceeds shot noise. Therefore, conditions to adjust the two cameras to reduce the noise when used in a balanced configuration are presented. It is shown that balance detection can reduce the noise in real time operation, in comparison with single camera configurations. However, simple deduction of an average spectrum in single camera configurations delivers less noise than the balance detection.

Sensitivity enhancement of fiber-laser-based stimulated Raman scattering microscopy by collinear balanced detection technique

We propose the collinear balanced detection (CBD) technique for noise suppression in fiber laser (FL)-based stimulated Raman scattering (SRS) microscopy. This technique reduces the effect of laser intensity noise at a specific frequency by means of pulse splitting and recombination with a time delay difference. We experimentally confirm that CBD can suppress the intensity noise of second harmonic (SH) of Er-FL pulses by 13 dB.The measured noise level including the thermal noise is higher by only ~1.4 dB than the shot noise limit. To demonstrate SRS imaging, we use 4-ps SH pulses and 3-ps Yb-FL pulses, which are synchronized subharmonically with a jitter of 227 fs. The effectiveness of the CBD technique is confirmed through SRS imaging of a cultured HeLa cell.

Differentially Encoded Photonic Analog-to-Digital Conversion Based on Phase Modulation and Interferometric Demodulation

We propose a novel approach to realizing photonic analog-to-digital conversion in a system with a phase modulator (PM) and a delay-line interferometer (DLI). The phase modulation and interferometric demodulation techniques are employed to realize quantization and encoding. In the system, the phase-modulated signal is optically sampled by a multiwavelength pulsed source and the phase-shifted transfer functions of the DLI under different wavelengths are used for encoding. In the approach, the differential signal, but not the original analog signal, is encoded, which increases the equivalent number of bits. In addition, the balanced detection technique can be incorporated into the system to alleviate the influences of amplitude and timing jitter.

Quantum correlations of pulses of optical parametric oscillator synchronously pumped above threshold

The quantum analysis of radiation from a degenerate optical parametric oscillator synchronously pumped above its oscillation threshold is presented. It is shown that pulses of signal and pump fields at the output of the oscillator have the following properties: quantum fluctuations of the fields are independent in each individual pulse, but correlated in pulses of the pulse train with a temporal step multiple of the pulse period. The number of essentially correlated pulses is on the order of the oscillator cavity finesse. Cross-correlations between the pump and signal pulses are established above the oscillation threshold. These correlations lead to a significant quantum effect in the integral characteristics of the fields. A theoretical analysis revealed that the spectrum of field fluctuations measured using a balanced homodyne detection technique of phase quadratures of the fields with a pulsed local oscillator reveals quantum noise suppression in the vicinity of frequencies that are multiples of the pulse repetition rate.

Dark fringe operation and noise reduction with optical subtraction approach and power allocation

We demonstrate a technique to achieve the reduction of excess noise with dark fringe operation by optical subtraction and power allocation strategy. By performing the subtraction in optical way, the excess noise resulted from the photocurrent is eliminated and the phase information can be extracted with single photoreceiver. Meanwhile, by allocating more optical power to the probe beam, the DC background and the signal amplitude of the photocurrent become very small simultaneously so that the dark fringe operation is achieved. We compare the spectral distributions resulted from the proposed technique with that from the widely employed balanced detection and the modified balanced detection techniques which both are based on electronic subtraction. It is found that the experimental results closely agree to the theoretical predictions.

Continuous variable tomographic measurements

Using a recent result of Albini et al. [P. Albini, E. De Vito, A. Toigo, J. Phys. A: Math. Theor. 42 (2009) 295302] to represent quantum homodyne tomography in terms of a single observable (as a normalized positive operator measure) we construct a generalized Markov kernel which transforms the measurement outcome statistics of this observable into the measurement outcome statistics of a covariant phase space observable. We also consider the inverse question. Finally, we add some remarks on the quantum theoretical justification of the experimental implementations of these observables in terms of balanced homodyne and 8-port detection techniques, respectively.

3×3 Mach-Zehnder interferometer with unbalanced differential detection for full-range swept-source optical coherence tomography

Quadrature interferometry based on 3x3 fiber couplers could be used to double the effective imaging depth in swept-source optical coherence tomography. This is due to its ability to suppress the complex conjugate artifact naturally. We present theoretical and experimental results for a 3x3 Mach-Zehnder interferometer using a new unbalanced differential optical detection method. The new interferometer provides simultaneous access to complementary phase components of the complex interferometric signal. No calculations by trigonometric relationships are needed. We demonstrate a complex conjugate artifact suppression of 27 dB obtained in swept-source optical coherence tomography using our unbalanced differential detection. We show that our unbalanced differential detection has increased the signal-to-noise ratio by at least 4 dB compared to the commonly used balanced detection technique. This is due to better utilization of optical power.

High-efficiency InGaAs QW distributed Bragg reflector laser with curved grating for squeezed light generation

A high-efficiency InGaAs quantum-well curved distributed Bragg reflector (DBR) laser was designed for squeezed light generation and fabricated using a selective-area quantum-well disordering technique to reduce absorption loss in the DBR grating region. Single-mode lasing with side-mode suppression ratio of 50 dB was obtained, and an external differential quantum efficiency as high as 0.71 was achieved under continuous-wave operation. Using balanced detection technique, the laser intensity noise relative to the standard quantum limit (SQL) was measured. Ultra-low noise characteristic of 0.8 dB below the standard quantum limit was achieved by wideband constant-current driving of the laser. This is the first demonstration of single-mode squeezed light generation by a monolithic semiconductor laser at room temperature.

Low-noise detection of ultracold atoms

We have demonstrated a new technique for detecting ultracold atoms. A balanced detection technique was used to reduce laser-induced detection noise in conjunction with modulation-transfer spectroscopy to distinguish cold atoms from a thermal cloud. Using this technique, we have achieved signal-to-noise ratios in excess of 2000:1.

Noise in optical low-coherence reflectometry

It is theoretically and experimentally shown that Fresnel end reflection at a waveguide under test can degrade the sensitivity of an optical low-coherence reflectometer (OLCR) with a balanced detection scheme. Optical mixing of the local oscillator (LO) light and the end reflection produces beat noise whose current noise spectral density is represented by 2(1+P/sup 2/)<I/sub 1/><I/sub 2/>//spl delta//spl nu/, where P and /spl delta//spl nu/ are the degree of polarization and the effective linewidth of the light, respectively, and <I/sub 2/> and <I/sub 2/> are the total mean photocurrents of the LO light and the end reflection at the balanced mixer, respectively. The balanced detection technique suppresses the intensity noise of the light and the beat noise becomes the dominant source of sensitivity degradation. The minimum detectable reflectivity is derived which includes the effect of sensitivity degradation caused by beat noise.

Wide band AC balanced homodyne detection of weak coherent pulses

A wide band AC balanced homodyne detection technique was applied to measure the amplitude of coherent pulses. The ultimate sensitivity given by a peak signal to noise ratio (S/N)=1 of our system is estimated to be about 0.25 fW (3.3×10 −24 J/pulse or a photon number of 1.25×10 −5 photons/pulse). As a demonstration for probing very weak ultrafast coherent optical processes, an exciton-polariton oscillation in a GaAs single quantum well microcavity was observed up to nine oscillation periods.

Optical corpuscular theory of semiconductor laser intensity noise and intensity squeezed-light generation

Fluctuations of the stored energy and the emitted power of a semiconductor laser are derived by classical corpuscular optical theory. The fundamental noise sources are the shot noise associated with a field’s conversion to emitting or absorbing atoms and the mirror loss noise. The latter is taken into account in the form of partition noise forces linked to laser facet reflection. The theory permits the description of the nonclassical states of light, and its results agree with quantum theory. For quiet pumping conditions and for a high-reflection coated Fabry–Perot laser, 50% of internal photon noise suppression is obtained, whereas nonfluctuating optical output is possible for negligible internal loss. The influence of gain suppression on amplitude squeezing is discussed. The effect of attenuation on the propagation of laser fluctuations is studied by use of the concept of optical partition noise. This leads to the invariance of a suitably defined relative intensity noise, which becomes negative for sub-Poissonian photon statistics. The setup for the intensity noise measurement with the balanced detection technique is analyzed by use of optical partition noise.

Ultrasensitive, visible tunable diode laser detection of NO_2

Recent advances in room-temperature visible diode lasers and ultrasensitive detection techniques have been exploited to create a highly sensitive tunable diode laser absorption technique for in situ monitoring of NO(2) in the lower troposphere. High sensitivity to NO(2) is achieved by probing the visible absorption band of NO(2) with an AlGalnP diode laser at 640 or 670 nm combined with a balanced ratiometric electronic detection technique. We have demonstrated a sensitivity of 3.5 × 10(10) cm(-3) for neat NO(2) in a 1-m path at 640 nm and have estimated a sensitivity for ambient operation of 5 ppbv m (l0 ppbv m at 670 nm), where ppbvm is parts in 10(9) by volume per meter of absorption path length, from measured pressure-broadening coefficients.

Three-dimensional displacement measurement of a tube scanner for a scanning tunneling microscope by optical interferometer

The scanning tunneling microscope (STM) is known for its high lateral resolution. The unreliability of the scanning and positioning motion of the STM were monitored by interferometer and capacitance recently. The widely used conventional lead zirconium titanate (PZT) tube scanner has good mechanical properties; however, the motion of the tube includes angular motion which generates large error. Even with an optical interferometer to calibrate STM tip motion, the angular motion reduces or eliminates the visibility of interference. We have developed an eight-segmented tube scanner which reduces angular motion for maintaining the visibility of interference and reduces Abbe error. The residual error of the value measured by a high-accuracy interferometer which measured the motion of the newly developed eight-segmented tube scanner was estimated by measuring a standard pattern. The accuracy of the interferometer, which used a balanced detection technique, the principle of which is also reported in this paper, was measured by a capacitance gauge.

Balanced detection technique to measure small changes in transmission

A novel technique to measure changes in optical transmission of better than 10(-3) with time resolution of better than 10 ns is discussed. The photocurrent from two photodiodes is subtracted with a balanced detection circuit to eliminate the common signal; carrier-induced change in absorption for a semiconductor single quantum well produces a large differential signal.

Fiber-optic DC magnetic field sensor with balanced detection technique

The authors describe a balanced detection technique that suppresses undesirable effects due to residual signals for the detection of DC and low-frequency magnetic fields with intermetric fiber-optic sensors using metallic glass. The detection technique utilizes a square-wave magnetic field modulation that translates the residual signal outside of the detection bandwidth. Experimental results are presented that show effective suppression of the residual signal.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Phase-noise and shot-noise limited operations of low coherence optical time domain reflectometry

This letter shows theoretically and experimentally that Fresnel end reflection of the waveguide under test degrades the sensitivity of low coherence optical time domain reflectometry (OTDR). Optical mixing of end reflection and reference light in the OTDR produces the phase noise in proportion to end reflectivity. With the balanced detection technique, the excess photon noise is subdued and the phase noise becomes the dominant source of sensitivity degradation. At 3.2% end reflection and 300 μA mean photocurrent, the noise floor due to the phase noise is −138 dB/Hz. By reducing the fiber end reflection with matching oil, a shot-noise limited sensitivity of −140 dB at a 3 Hz bandwidth has been demonstrated at submillimeter resolution.