Wavelength modulation spectroscopy: combined frequency and intensity laser modulation

A theoretical model of wavelength modulation spectroscopy that uses a laser diode on a Lorentzian absorption line is presented. This theory describes the general case of a current-modulated semiconductor laser, for which a combined intensity and frequency modulation with an arbitrary phase shift occurs. On the basis of this model, the effect of several modulation parameters on the detected signals is evaluated. Experimental signals measured on an absorption line of CO2 by use of a 2-microm distributed-feedback laser are also presented and validate this analysis. These experimental results agree with the calculated signals, confirming the relevance of the model.     

Spectral measurement of frequency modulation index

A method is proposed of measuring the frequency modulation index based on the evaluation of the amplitudes of four spectral lines. The method does not require a priori knowledge of the harmonic numbers of spectral components of frequency-modulated signals. The accuracy of the proposed method is evaluated. Translated from Izmeritel'naya Tekhnika, No. 9, pp. 37–40, September, 1998.     

The direct measurement of linewidth using an atomic force microscope

A method for directly measuring linewidth with an atomic force microscope using the first derivative of the signal is presented. The method showed that it is possible to make a direct measurement of the size of the upper base of a trapezoidal protrusion down to 30 nm. __________ Translated from Izmeritel’naya Tekhnika, No. 5, pp. 10–12, May, 2008.     

Application of modulation spectroscopy for determination of recombination center parameters

The authors have shown the possibility to identify both type of light-emitting transitions and parameters of recombination centers using differential luminescence spectra for wide-gap II-VI and III-V semiconductors.     

Dynamic-transmit focusing using time-dependent focal zone and center frequency

A method for achieving dynamic-transmit focus is presented. Within an initial high bandwidth pulse, successively higher frequencies are focused to successively closer focal zones along a transmitted beam line. During the receive operation, a time varying bandpass filter initially passes the higher frequencies that are focused closer in and successively passes lower center frequencies as time evolves and the focal zone moves out. The dynamically focused receive pulses are digitally sampled and processed by a matched digital filter to minimize phase anomalies. In this way, an improved resolution image is obtained with no loss of frame rate. The method is evaluated using comprehensive simulations that account for realistic levels of phase aberration and tissue attenuation. The method is relatively robust with respect to these perturbations. When the appropriate conditions apply, the new method can achieve an improvement in mean lateral resolution similar to that found in a multiple transmit zone implementation but without the frame rate penalty. A discussion of implementation considerations and limitations is presented.     

High-frequency intensity modulation in orthogonal polarized dual frequency lasers with optical feedback

High-frequency modulation of laser output intensity is studied with asymmetric feedback induced by the misalignment of an external feedback reflector in an orthogonal polarized dual frequency laser. The fringe frequency of the optical feedback system is seven times higher than that of a conventional optical feedback system, due to multiple feedback effects. The output characteristics of two orthogonal polarized modes are also investigated. Mode competition is observed between the two modes. When initial intensities of the two modes are unequal, the mode competition will be strong. The difference in initial intensity between the two orthogonally polarized modes plays an important role in the mode competition with optical feedback. Experimental results are presented, as well as a theoretical explanation. The high-frequency modulation of laser intensity can greatly increase the resolution of an optical feedback sensing system.     

Beat frequency measurement system for multiple dual polarization fiber DFB lasers

This paper describes the design and testing of an electronic system intended to record the beat frequency signal from up to eight serially multiplexed dual polarization fiber distributed feedback lasers. The beat frequency from each laser is first mixed down to around 70 MHz and then measured using a PC based counter system. The gate frequency and, hence, the sampling rate, is designed to be around 1 kHz, and the frequency resolution of the system is found to be equal to the gate frequency. The system has been used to measure the beat frequency variation of fiber lasers due to either rapidly varying temperature or flexing motion. Measurements made with multiple fiber laser sensors show that the crosstalk between different sensors is very low.     

Simultaneous measurement of phonons and ionization using SiCAD s

Our group at Stanford has configured a double-sided Silicon Crystal Acoustic Detector (SiCAD) for simultaneous measurement of both phonons and ionization. This detector operates at 370mK and consists of Ti Transition Edge Sensors (TES), having microsecond resolution times, patterned on both sides of a 300µm thick Si wafer. Distinguishing an electron-recoil event from a nuclear-recoil event is possible due to the different partitioning of energy into phonons vs. ionization for the two types of event. This is a powerful background rejection technique for neutrino physics experiments and dark matter searches where the events of interest are nuclear-recoils. In addition, the phonon sensor is position sensitive. In particular, it determines the depth of an event and can be used to reject events occurring near the surface. This detector also allows us to refine our recent measurement of the fraction of phonon energy propagating ballistically from nuclear-recoil events¹.     

Two-tone frequency modulation spectroscopy from laser light scattered off a hard target

One can apply two-tone frequency modulation spectroscopy techniques to the detection of gas-phase species by using laser light scattered from hard targets. High sensitivities are demonstrated, with a minimum detectable absorption of 10(-4) possible with a simple apparatus. The effects of laser speckle on the FM signal are described, and we show that the detection signal-to-noise ratio can be improved by collecting an increased number of speckle cells.     

Mapping nanoscale elasticity and dissipation using dual frequency contact resonance AFM

We report on a technique that simultaneously quantifies the contact stiffness and dissipation of an AFM cantilever in contact with a surface, which can ultimately be used for quantitative nanomechanical characterization of surfaces. The method is based on measuring the contact resonance frequency using dual AC resonance tracking (DART), where the amplitude and phase of the cantilever response are monitored at two frequencies on either side of the contact resonance. By modelling the tip-sample contact as a driven damped harmonic oscillator, the four measured quantities (two amplitudes and two phases) allow the four model parameters, namely, drive amplitude, drive phase, resonance frequency and quality factor, to be calculated. These mechanical parameters can in turn be used to make quantitative statements about localized sample properties. We apply the method to study the electromechanical coupling coefficients in ferroelectric materials and the storage and loss moduli in viscoelastic materials.     

Local oscillator limited frequency stability for passive atomic frequency standards using square wave frequency modulation

Atomic frequency standards using square wave frequency modulation effectively interrogate the atomic line by switching back and forth between two frequencies with equal atomic absorption values. For a symmetric absorption line, the slope of the responses will also be equal. In the quasistatic limit, this would seem to be an ideal interrogation process: the sign reversal of frequency slope can be removed by detection electronics to give an essentially unvarying sensitivity to local oscillator frequency variations. Such an interrogation would seem to eliminate L.O. aliasing and relieve stringent requirements on L.O. phase noise. Nevertheless, sign changes in the interrogation and detection processes mean that the sensitivity is actually zero at some point in the cycle. We derive consequences of this fact by an analysis in terms of the sensitivity function g(t). For white phase noise, we derive an optimal form for g(t) and show that the aliased noise always diverges as g(t) approaches a constant. For flicker phase noise, we find a limiting form that could, in principle, eliminate the aliasing effect; in practice, however, the improvement is limited by a slow dependence on available bandwidth. Finally, we derive optimized forms for any phase noise spectrum.     

Doppler-free spectroscopy using a continuous-wave optical frequency synthesizer

A continuous-wave (cw) optical frequency synthesizer is demonstrated by using a monolithic-type cw optical parametric oscillator (cw-OPO) and an optical frequency comb. The cw-OPO is phase locked to an optical frequency comb that is phase locked to an atomic clock. The output frequency of the cw-OPO is frequency shifted with an electro-optic modulator, which makes it possible to tune the frequency continuously over 10 GHz. Furthermore, Doppler-free spectroscopy is performed using the optical frequency synthesizer for a cesium D1 line at 895 nm. The observed linewidth of 5 MHz is the natural linewidth of cesium. The center frequency of the line is consistent with a previous report.     

Simultaneous wavenumber measurement and coherence detection using temporal phase unwrapping

Wavelength scanning interferometry and swept-source optical coherence tomography require accurate measurement of time-varying laser wavenumber changes. We describe here a method based on recording interferograms of multiple wedges to provide simultaneously high wavenumber resolution and immunity to the ambiguities caused by large wavenumber jumps. All the data required to compute a wavenumber shift are provided in a single image, thereby allowing dynamic wavenumber monitoring. In addition, loss of coherence of the laser light is detected automatically. The paper gives details of the analysis algorithms that are based on phase detection by a two-dimensional Fourier transform method followed by temporal phase unwrapping and correction for optical dispersion in the wedges. A simple but robust method to determine the wedge thicknesses, which allows the use of low-cost optical components, is also described. The method is illustrated with experimental data from a Ti:sapphire tunable laser, including independent wavenumber measurements with a commercial wavemeter. A root mean square (rms) difference in measured wavenumber shift between the two of ~4 m?1 has been achieved, equivalent to an rms wavelength shift error of ~0.4 pm.     

Convolutional coded dual header pulse interval modulation for line of sight photonic wireless links

The analysis and simulation for convolutional coded dual header pulse interval modulation (CC-DH-PIM) scheme using a rate 1/2 convolutional code with the constraint length of 3 is presented. Decoding is implemented using the Viterbi algorithm with a hard decision. Mathematical analysis for the slot error rate (SER) upper bounds is presented and results are compared with the simulated data for a number of different modulation techniques. The authors show that the coded DH-PIM outperforms the pulse position modulation (PPM) scheme and offers >4 dB code gain at the SER of 10^-4 compared to the standard DH-PIM. Results presented show that the CC-DHPIM with a higher constraint length of 7 offers a code gain of 2 dB at SER of 10^{-5 compared to the CC-DH-PIM with a constraint length of 3. However, in CC-DH-PIM the improvement in the error performance is achieved at the cost of reduced transmission throughput compared to the standard DH-PIM.}     

Filterless frequency octupling circuit using dual stage cascaded polarization modulators

This paper demonstrates the photonic generation of millimetre-wave carriers by frequency octupling of a radio frequency (RF) carrier using a circuit with two parallel arms each containing two polarization modulators in series. The low RF drive power required and the absence of optical filters make this architecture suitable for generating highly stable and tunable optical millimetre-waves. A transfer function representation is used to analyse the operation and a simulation using Virtual Photonic Inc. software package is presented as a proof of concept. The simulation results show very high unwanted optical and electrical side harmonic suppression ratio for a wide range of modulation index compared to the prior art. Further, considering the practical scenario, operating tolerance against the imbalances in the key parameters is established. The unwanted RF sidebands are more than 19?dB and 30 dB below the desired signal power even when a deviation of ±3° is considered in individual RF phase and polarizer angle respectively.