Ultimate linewidth reduction of a semiconductor laser frequency-stabilized to a Fabry-Pérot interferometer

We report a theoretical dynamical analysis on effect of semiconductor laser phase noise on the achievable linewidth when locked to a Fabry-Perot cavity fringe using a modulation-demodulation frequency stabilization technique such as the commonly used Pound-Drever-Hall frequency locking scheme. We show that, in the optical domain, the modulation-demodulation operation produces, in the presence of semiconductor laser phase noise, two kinds of excess noise, which could be much above the shot noise limit, namely, conversion noise (PM-to-AM) and intermodulation noise. We show that, in typical stabilization conditions, the ultimate semiconductor laser linewidth reduction can be severely limited by the intermodulation excess noise. The modulation-demodulation operation produces the undesirable nonlinear intermodulation effect through which the phase noise spectral components of the semiconductor laser, in the vicinity of even multiples of the modulation frequency, are downconverted into the bandpass of the frequency control loop. This adds a spurious signal, at the modulation frequency, to the error signal and limits the performance of the locked semiconductor laser. This effect, reported initially in the microwave domain using the quasistatic approximation, can be considerably reduced by a convenient choice of the modulation frequency.     

Measurement of the linewidth of a continuous-wave laser with a cavity-length modulation technique

We introduce a Fabry-Perot cavity-length modulation technique for measuring the linewidth of a continuous wave (cw) laser. We calculate the peak intensity of a cw laser transmitted through a Fabry-Perot cavity as a function of mirror speed. By fitting the experimental data to the results of the calculation, we determine the linewidth of a frequency-stabilized cw laser. The linewidth of a cw ring dye laser measured in the 570-590-nm wavelength range is approximately 170 +/- 20 kHz. We also demonstrate the use of this technique to measure the reflectivity of a high-reflectance mirror.     

Frequency- and intensity-noise measurements of a widely tunable 2-/spl mu/m Tm-Ho:KYF laser

The measurement of the frequency and intensity noise in a novel single-mode 2-/spl mu/m Tm-Ho:KYF laser is presented. The laser frequency noise is measured by exploiting the fringe side of the transmission of a Fabry-Pe/spl acute/rot interferometer. The measured power spectral density of the frequency noise is principally characterized by a random-walk noise contribution, which sets an emission linewidth of /spl sim/ 600 kHz for the 2-/spl mu/m radiation. The relative intensity noise (RIN) reaches the quantum limit of -155 dB/Hz for Fourier frequencies above 1 MHz and shows a maximum level of -90 dB/Hz at the relaxation-oscillation frequency of 20 kHz.     

Absolute frequency stability of a diode-laser-pumped Nd:YAG laser stabilized to a high-finesse optical cavity

We report the frequency stabilization of a diode-laser-pumped monolithic ring Nd:YAG laser locked to a high-finesse optical cavity. With an independent cavity as a frequency discriminator, the absolute frequency noise was measured to be as low as 2 × 10(-2) Hz/Hz(1/2) at the Fourier frequency of approximately 3 kHz. We also measured the heterodyne beat note between two lasers locked to the independent cavities. The beat linewidth is narrower than 30 Hz and the minimum root Allan variance is approximately 6 × 10(-14).     

Phase filtering in nonlinear-imaging technique with a phase object

Using the nonlinear-imaging technique with a phase object (NIT-PO), we have studied third-order nonlinearities of various samples. In this work, we develop, for pure nonlinear refractive materials, an approximate method to calculate the nonlinear refractive coefficient analytically. By decomposing the object field passing through the phase object into two top-hat beams of different phases and beam radius, we acquire the approximate phase contrast, from which we extract the nonlinear refractive coefficient. This approximation is valid when the on-axis nonlinear phase shift by the sample is less than π. In addition, this approximation serves to estimate the sensitivity and monotonic interval for nonlinearity measurements more easily and thus helps us to maximize both the sensitivity and monotonic interval of measurements. We test this method with CS(2), a well-characterized third-order nonlinear refractive material using 21 ps laser pulses at 532 nm. We expect this method can be applied to high-order nonlinear refraction cases.     

Influence of grating parameters on the linewidths of external-cavity diode lasers

We investigate experimentally the influence of the grating reflectivity, grating resolution, and diode facet antireflection (AR) coating on the intrinsic linewidth of an external-cavity diode laser built with a diffraction grating in a Littrow configuration. Grating lasers at 399, 780, and 852 nm are determined to have typical linewidths between 250 and 600 kHz from measurements of their frequency noise power spectral densities. The linewidths are little affected by the presence of an AR coating on the diode facet but narrow as the grating reflectivity and grating resolution are increased, with the resolution exerting a greater effect. We also use frequency noise measurements to characterize a laser mount with improved mechanical stability.     

Near shot-noise-level relative frequency stabilization of a laser-diode-pumped Nd:YVO(4) microchip laser

A laser-diode-pumped Nd:YVO(4) microchip laser was built and actively frequency stabilized relative to a Fabry-Perot cavity with the frequency-modulated sideband technique. The error signal reaches the shot-noise level of 7.4 mHz/√Hz around 1 kHz. Excess intensity noise sets a lower limit of16.5 mHz/√Hz for the relative frequency noise, corresponding to a spectral linewidth of 860 μHz. We discuss the method for reconstructing the actual frequency deviation from the observed error signal.     

Two-level atom in a squeezed vacuum with finite bandwidth

Abstract

The resonance fluorescence of a two-level atom driven by a coherent laser field and damped by a finite bandwidth squeezed vacuum is analysed. We extend the Yeoman and Barnett technique to a non-zero detuning of the driving field from the atomic resonance and discuss the role of squeezing bandwidth and the detuning in the level shifts, widths and intensities of the spectral lines. The approach is valid for arbitrary values of the Rabi frequency and detuning but for the squeezing bandwidths larger than the natural line-width in order to satisfy the Markoff approximation. The narrowing of the spectral lines is interpreted in terms of the quadrature-noise spectrum. We find that, depending on the Rabi frequency, detuning and the squeezing phase, different factors contribute to the line narrowing. For a strong resonant driving field there is no squeezing in the emitted field and the fluorescence spectrum exactly reveals the noise spectrum. In this case the narrowing of the spectral lines arises from the noise reduction in the input squeezed vacuum. For a weak or detuned driving field the fluorescence exhibits a large squeezing and, as a consequence, the spectral lines have narrowed linewidths. Moreover, the fluorescence spectrum can be asymmetric about the central frequency despite the symmetrical distribution of the noise. The asymmetry arises from the absorption of photons by the squeezed vacuum which reduces the spontaneous emission. For an appropriate choice of the detuning some of the spectral lines can vanish despite that there is no population trapping. Again this process can be interpreted as arising from the absorption of photons by the squeezed vacuum. When the absorption is large it may compensate the spontaneous emission resulting in the vanishing of the fluorescence lines.     

Analysis of light noise sources in a recycled Michelson interferometer with Fabry-Perot arms

We present a method by which the effect of laser field variations on the signal output of an interferometric gravitational wave detector is rigorously determined. Using the Laser Interferometer Gravitational Wave Observatory (LIGO) optical configuration of a power recycled Michelson interferometer with Fabry-Perot arm cavities as an example, we calculate the excess noise after the input filter cavity (mode cleaner) and the dependence of the detector strain sensitivity on laser frequency and amplitude noise, radio frequency oscillator noise, and scattered-light phase noise. We find that noise on the radio frequency sidebands generally limits the detector's sensitivity.     

Microwatt shot-noise measurement

We report a simple scheme for sensitive measurements of optical-noise spectra. Optical noise is separated from electronic noise when the output of an analog spectrum analyzer is real-time squared and then lock-in detected. This method directly yields the desired mean-square noise voltage, i.e., the power spectrum of the optical noise on a linear scale. To demonstrate this technique, the mean-square shot noise of a laser beam is measured and found to vary linearly with the laser power from several milliwatts down to one microwatt, in excellent quantitative agreement with predictions.     

测量激光线宽的一种新装置

介绍了一种新的仅用一台激光器测定激光线宽或频率稳定的测量方法，用稳定的无源腔透过曲线腰处斜率将激光的频率扰动转为电压扰动。用所研制的测量装置对自制的边疆波染料激光频率的线宽和频率稳定性进行了测量。     

Frequency stabilization of a 1.3 microm laser diode using double resonance optical pumping in the 5P 3/2-6S 1/2 transition of Rb atoms

The frequency stabilization of a laser diode in the 1.3 microm region using double resonance optical pumping (DROP) spectrum in the 5P(3/2)-6S(1/2) transition of (87)Rb atoms is demonstrated. The signal-to-noise ratio of the DROP spectrum is approximately ten times higher than that of the previous optical-optical double resonance spectrum. The spectral linewidth of the DROP measures 8.4 MHz. When the frequency of a 1.367 microm laser diode is stabilized to the DROP spectrum, the frequency stability is 9 x 10(-12) after 100 s. Also, the wavelength of the frequency-stabilized laser locked to the 5P(3/2)-6S(1/2) transition using a wavelength meter is measured.     

Self-aligned extended-cavity diode laser stabilized by the Zeeman effect on the cesium D2 line

An extended-cavity diode laser at 852 nm has been built especially for the purpose of cooling and probing cesium atoms. It is a compact, self-aligned, and continuously tunable laser source having a 100-kHz linewidth and 60-mW output power. The electronic control of the laser frequency by the piezodriven external reflector covers a 4.5-kHz bandwidth, allowing full compensation of acoustic frequency noise without any adverse effect on the laser intensity noise. We locked this laser to Doppler-free resonances on the cesium D(2) line by using the Zeeman modulation technique, resulting in the frequency and the intensity of the laser beam being unmodulated. We also tuned the locked laser frequency over a span of 120 MHz by using the dc Zeeman effect to shift the F = 4-F' = 5 reference transition.     

Simple method for frequency locking of an extended-cavity diode laser

We have developed an extended-cavity tunable diode laser system that has a small linewidth and a large output power (more than 90% of the free-running power) whose operating frequency can be conveniently locked to a transition line of Rb atoms. Based on flat-mirror feedback and frequency self-locking and with weak feedback, we have achieved a continuous frequency detuning range greater than 900 MHz and a short-time linewidth stability of better than 0.4%. By using a two-step locking procedure we not only can lock the laser frequency but also can detune the frequency to any desired value. The locking is quite sturdy and rugged.     

Stabilizing the spectrum of a femtosecond Ti:Sapphire laser

A new scheme for stabilizing the beat frequency of the interacting longitudinal modes in a femtosecond Ti:sapphire laser (Femos-1) has been developed and experimentally studied. The stability and frequency spectrum of mode beats in a modified laser have been monitored. A short-term instability of the beat frequency does not exceed 4.8 × 10^?8 (τ = 10 s) over a time scale of 10 min. The laser linewidth in the regime of self-mode-locking is ≤3 Hz at a laser spectrum width of 20 nm.     

Noise-Enhanced Measurement of Weak Doublet Spectra with a Fourier-Transform Spectrometer and a 1-Bit Analog-to-Digital Converter

We demonstrate an efficient noise dithering procedure for measuring the power spectrum of a weak spectral doublet with a Fourier-transform spectrometer in which the subthreshold interferogram is measured by a 1-bit analog-to-digital converter without oversampling. In the absence of noise, no information is obtained regarding the doublet spectrum because the modulation term s(x) of its interferogram is below the instrumental detection limit B, i.e., |s(x)| < B, for all path difference x values. Extensive numerical experiments are carried out concerning the recovery of the doublet power spectrum that is represented by s(x) = (s(0)/2)exp(-pi(2)x(2)/beta)[cos(2pif(1)x) + cos(2pif(2)x)], where s(0) is a constant, beta is the linewidth factor, and ?f? = (f(1) + f(2))/2. Different values of ?f?, s(0), and beta are considered to evaluate thoroughly the accuracy of the procedure to determine the unknown values of f(1) and f(2), the spectral linewidth, and the peak values of the spectral profiles. Our experiments show that, even for short observation times, the resonant frequencies of s(x) could be located with high accuracy over a wide range of ?f? and beta values. Signal-to-noise ratios as high as 50 are also gained for the recovered power spectra. The performance of the procedure is also analyzed with respect to another method that recovers the amplitude values of s(x) directly.     

Temperature dependence of the Brillouin linewidth in water

In the frequency spectrum of light that is scattered in liquid water there is a central elastically scattered peak that is due mainly to scattering by suspended particles; and, there is a peak on each side of the central peak that is displaced by the Brillouin frequency shift. The Brillouin shift is a direct measure of sound speed. The linewidth of the Brillouin shifted lines is dependent on the bulk and shear viscosity of water as well as its density, thermal conductivity, and specific heat. The linewidth of the Brillouin peaks has been investigated in laboratory experiments over a range 1°C to 35°C. The frequency spectrum of back-scattered laser light was analysed using a scanning Fabry-Perot etalon. A strong dependence of the linewidth of the Brillouin shifted lines on temperature was found. In particular, for low temperatures in the range 10°C down to 1°C the linewidth shows an increase from 750 MHz to 1.4 GHz.     

Application of superresolution techniques to ring laser gyroscopes: exploring the quantum limit

Two methods for extracting linewidth estimates from evenly sampled partially coherent signals, where white frequency fluctuations are the major source of noise, are discussed. The first method utilizes the Allan deviation, requiring multiple frequency estimates for each linewidth estimate. The second method utilizes the second-order autoregressive model to provide a linewidth estimate with each frequency estimate. A characterization shows that the latter technique is reliable for short data sets (相似文献   

Tunable and frequency-stabilized diode laser with a Doppler-free two-photon zeeman lock

We describe frequency locking of a diode laser to a two-photon transition of rubidium using the Zeeman modulation technique. We locked and tuned the laser frequency by modulating and shifting the two-photon transition frequency with ac and dc magnetic fields. We achieved a linewidth of 500 kHz and continuous tunability over 280 MHz with no laser frequency modulation.     

Effects of reference laser intensity noise in a Fourier transform spectrometer

Many noise sources deteriorate the performance of a Fourier-transform spectrometer. When such an instrument utilizes a reference source such as an HeNe laser to sample the measurement data, it is desirable to determine how the intensity fluctuations of the reference source will affect the final spectrum. This text shows how the intensity noise of the reference laser is transformed at the output of the interferometer, how it translates into a sampling jitter by a simple linear relation under a small bandwidth approximation, and how it is manifested in the final spectrum as a noise floor. Analytical and simulation results show that for an instrument designed for optical communication applications, the intensity noise of the reference laser may be one of the dominant noise source limiting the overall performance of the instrument.