Laser Frequency Fluctuations Research Articles

Frequency instability in Er/Yb fiber grating lasers due to heating by nonradiative transitions

Heating of the Er/Yb-codoped fiber lasers due to strong pump absorption was found to cause large frequency fluctuation in addition to lasing wavelength shift. Laser frequency fluctuation of up to 1.7 GHz was measured by using an absorption line of an acetylene gas cell. The large frequency fluctuation was reduced to less than 80 MHz with proper cooling of the laser.

Measurement of the frequency fluctuations of a laser with a nonlinear-absorbing cell using the Bershtein method

We propose to use the method of I. L. Bershtein for measuring the frequency fluctuations of an arbitrary type oscillator from the error signal at the output of the phase locking system for a laser with internal absorbing cell. The spectral density of the frequency fluctuations of a single-frequency He-Ne/CH4 laser was measured. It is shown that in this case the method allows us to measure the low-frequency technical fluctuations of the laser frequency, which determine the width of the laser emission line. The measurements can be performed in the case of both suppression of low-frequency fluctuations of the laser frequency due to the phase locking system and the regime that is close to that of free generation. It is also shown that an increase in the operating frequency band of the phase locking system makes sense only in the case in which the reference spectral density of the laser frequency fluctuations multiplied by the slope of the frequency discriminator conversion (in this case, the power and photoreceiver peak) exceeds the noise level at the system input.

Closed-loop phase-shifting interferometry with a laser diode

A closed-loop phase-shifting Fizeau-type interferometer at lambda=633 nm was constructed with the direct frequency modulation of laser diodes. The interferometer is servo controlled fully in the phase domain where optical phases are detected by a two-frequency optical heterodyne method. Stepwise phase shifting was accomplished with good stabilization against external disturbances (vibration, air flow, etc.) and laser frequency fluctuations.

Improved multistage wide band laser frequency stabilization

Suppression of laser frequency fluctuations is an essential technology for planned interferometric detectors for astrophysical gravitational waves. Because of the low degree of residual frequency noise which is ultimately required, control topologies comprising two or more cascaded loops are favored. One such topology, used in the Laser Interferometer Gravitational-Wave Observatory 40 m interferometer, relied on electro-optic Pockels cell phase correction as a fast actuator for the final stage. This actuation method proved susceptible to spurious amplitude modulation effects, which provided an unintended parasitic feedback path. An alternate arrangement, which achieves comparably effective frequency stabilization without using a phase correcting Pockels cell, was introduced and successfully tested.

Influence of laser phase and frequency fluctuations on photon-echo data erasure

Erasure of data stored by use of photon echoes has been investigated as a function of data writing time and data storage time. The results clarify the requirements on laser phase and frequency stability for performing photon-echo data erasure. The analysis of phase and frequency stability of a light source by the photon-echo erasure process is illustrated. A theoretical analysis emphasizing the physical processes that affect the erasure efficiency as well as an extensive discussion of possible error sources are given. Finally, an approach to bit-selective photon-echo data erasure that is insensitive to laser phase and frequency fluctuations is suggested.

Frequency Chain to 3.39 µ mCH4-Stabilized He–Ne Laser Using Josephson Point Contact as Harmonic Mixer

A frequency-multiplication chain to the 3.39 µ m CH4-stabilized He–Ne laser has been constructed using the Josephson point contact as a harmonic mixer. The Josephson harmonic mixer simplifies the chain because it can directly connect the 90 GHz microwave and the 4.25 THz far-infrared frequencies. Tracking oscillators, which track the beat notes by phase locking and regenerate clean signals, also have been developed to correctly count the beat frequencies. The frequency fluctuations of free-running lasers are canceled by counting the beat frequencies with coincident gate timing. The absolute frequency measurement of the He–Ne laser is reported with a standard deviation of about 5 kHz (τ=8 s).

Diode-laser noise spectroscopy of rubidium

We report on spectra obtained by measuring the laser intensity noise after a broad-bandwidth diode-laser beam passes through a rubidium vapor cell. The atomic resonance converts laser frequency fluctuations into intensity fluctuations. We compare our experimental spectra with numerically calculated spectra based on a phase-diffusion model of the laser field and find good agreement.

Analysis of the noise sources in an optically pumped cesium beam resonator

The main noise sources that affect the clock signal detection in an optically pumped cesium beam resonator are determined and their influence on the clock S/N ratio is evaluated. The following noise sources are taken into account: atomic shot noise, fluorescence photon noise, laser frequency noise, and clock signal detection noise. A theoretical model that predicts the variations of the S/N ratio as a function of different parameters characterizing the atom-laser interaction is developed. The main conclusion concerns the saturation of S/N value for high atomic flux due to laser frequency fluctuations. All the theoretical predictions were experimentally verified.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Self-quenching of the semiconductor laser linewidth below the Schawlow–Townes limit by using optical feedback

We demonstrate theoretically and experimentally self-quenching of the fundamental semiconductor laser frequency fluctuations to a level that is orders of magnitude below the Schawlow-Townes limit for a solitary laser. It is shown that the main operative mechanism is the combined action of a frequency-dependent internal loss and amplitude-to-phase coupling. The internal frequency-dependent loss is introduced by means of spectrally narrow external optical feedback, which provides a strong frequency-dependent dispersion. Linewidth reduction by a factor of 2 x 10(3) is demonstrated by using a narrow Doppler-free Faraday resonance in Cs vapor.

Delayed-self-heterodyne measurement of laser frequency fluctuations

The two-sample (or Allan) variance of frequency fluctuations was used for laser frequency-noise characterization at Fourier frequencies below 1 MHz. The delayed self-heterodyne technique was used for the variance measurement. Analytical calculations ensure that just twice the variance values were observed at tau >

Measurement of the statistics of DFB laser frequency fluctuations

The statistics of the frequency of the bandpass filtered beat signal between a DFB-laser and an external cavity laser were measured. The distribution of frequency fluctuations from the centre frequency is shown to be very close to Gaussian, with the variance given by the product of beat signal linewidth and IF-filter bandwidth.

Non-Gaussian statistics of frequency fluctuations in line-narrowed semiconductor lasers

The statistics of the instantaneous frequency fluctuations of CW narrowed-linewidth single-mode semiconductor lasers are studied under the most general conditions. In the case in which the rate equations describing the evolution of the system cannot be linearized the statistics are found to deviate appreciably from a Gaussian distribution. A Monte Carlo simulation is shown to give results in good agreement with previous experimental measurements. >

Frequency offset locking of AlGaAs semiconductor lasers

Frequency offset locking was proposed as a reliable electrical negative feedback technique for tracking and sweeping of a semiconductor laser frequency. A frequency stabilized laser was used as a master laser, whose residual frequency fluctuations were 140 (kHz) at the integration time (τ) of 100 ms <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\leq \tau \leq 100</tex> s. A digital phase comparator of a large dynamic range of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2\pi \times 2^{11}</tex> (rad) was employed in the feedback loop to reduce the phase fluctuations of the beat signal between the master and slave lasers. Performances of frequency tracking and sweeping of the slave laser were quantitatively evaluated, and the results are: residual frequency fluctuations of the beat signal were reduced as low as 11 (Hz) at <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\tau = 100</tex> s, which meant that the residual frequency fluctuations of the slave laser were almost equal to those of the master laser, i.e., the slave laser frequency tracked accurately to the master laser frequency. Both the capture range and lock range of the beat frequency were 1.22 GHz. Frequency tunable range of the slave laser was 36.6 GHz under the condition of frequency offset locking, in which the slave laser frequency fluctuations were maintained as low as the one given above.

Statistics of laser frequency fluctuations in coherent optical receivers

We present a simple model for the statistics of the measured IF frequency in coherent optical receivers, for the case of major interest where the IF bandwidth is large compared with the linewidths of the signal and local oscillator lasers. We show that the PDF for the measured IF frequency is close to Gaussian in this case, with variance depending on both laser and filter characteristics.

Saturation of an atomic transition by a phase-diffusing laser field.

We have studied the effect of well-characterized laser frequency fluctuations on the saturation of an atomic resonance in a double optical resonance experiment. The peak-height asymmetry of the observed Autler-Townes signal was reversed at small detunings when the shape of the saturating laser power spectrum was changed from nearly Gaussian to nearly Lorentzian. This behavior agrees qualitatively with theoretical calculations and also with previous observations using lasers without such well-characterized fluctuations.

Flicker noise in frequency fluctuations of lasers.

Measured frequency fluctuation spectra ${S}_{\ensuremath{\delta}f}$ in ring lasers show that ${S}_{\ensuremath{\delta}f}=\frac{{h}_{\ensuremath{-}1}}{\ensuremath{\nu}}+\frac{{h}_{\ensuremath{-}2}}{{\ensuremath{\nu}}^{2}}$ at low Fourier frequencies $\ensuremath{\nu}$, while white noise prevails at higher frequencies. Experimentally, ${S}_{\ensuremath{\delta}f}=(\frac{\overline{A}{f}_{0}^{2}}{{Q}^{4}})(\frac{1}{\ensuremath{\nu}})$ has been found for the dependence of the $\frac{1}{\ensuremath{\nu}}$ noise (flicker noise) on the quality factor $Q$, with $\overline{A}\ensuremath{\approx}4$ (${f}_{0}=\mathrm{laser}\mathrm{frequency}$). The ${Q}^{\ensuremath{-}4}$ dependence is readily explained by loss (or gain) fluctuations, through use of a Van der Pol oscillator.

Phase noise characteristics of single mode semiconductor lasers with optical feedback

The phase noise characteristics of single mode semiconductor lasers with optical feedback have been studied. A correlation is found between the suppression of the low-frequency noise and the enhancement of the high-frequency noise near the relaxation oscillation frequency. Contrary to previous analyses, the low-frequency noise is minimized when the reflected light is 70°–90° out of phase with respect to that inside the diode laser cavity. A calculation of the laser frequency fluctuation spectrum, taking into account the amplitude-phase coupling of the laser field, agrees with the observation. A new explanation is proposed for the feedback-induced phase noise reduction effect.

Allowance for the influence of the modulation effect on measurements of laser frequency fluctuations

When laser frequency fluctuations are measured by the heterodyne method, frequency modulation results in an additional deterioration of the stability and such deterioration depends on the modulation frequency, frequency deviation, and averaging time. It is shown that the influence of modulation can be ignored if the averaging time is synchronized with the modulation frequency.

Lifetime determinations and their errors using pulsed dye lasers

The effects of the frequency fluctuations of pulsed dye lasers on the determination of atomic lifetimes by the decay of fluorescence are estimated. A small shift in the estimated lifetime will occur due to the inability to completely correct for pile-up errors. However, the statistics of the number of counts in each channel of the pulse height analyser are not affected to a first approximation.

Laser frequency fluctuations due to mechanical vibrations

The frequency fluctuations of a single-frequency laser due to mechanical vibrations are dependent on the sound pressure and sound frequency. The two dependences have been studied with the observation of beat signals between two stable external-mirror He-Ne lasers in the free-running condition. The dependence on the sound pressure follows Hooke's law, so that the difference between the frequency fluctuations due to the acoustic noises and those due to the external vibrations was quantitatively distinguished. Some causes of mechanical instability of the laser were clarified from the dependence on the sound frequency as compared with the lowest natural frequencies of each mechanical element in the cavity. The translation of mirrors along the optical cavity axis and two rotations of mirrors about the axis perpendicular to the optical cavity axis, which give rise to the variation in the relative angle between the laser beam axis and the Brewster window plate, are the most dominant motions that cause frequency fluctuations of the laser.