Measurement of Optical Frequency Stability by Using Spectral-Hole Burning

Techniques in frequency stabilization of lasers to fixed-spacer optical cavities have advanced to the point where the ultimate frequency stabilities are limited by thermal noise for standard cavity configurations at room temperature. In early experiments, laser stabilization via spectral-hole burning (SHB) has been shown to be a promising alternative. In this Letter, we explore the thermal-noise limits to frequency stability in spectral holes and compile known material parameters for a typical system used in SHB experiments (Eu3+ doped Y2SiO5) to make numerical estimates for the fundamental thermal-noise induced frequency instability in spectral holes for the liquid-helium temperature and dilution temperature cases. These efforts can guide the design of future SHB experiments and clarify which important material parameters remain to be measured.

The differential absorption lidar (DIAL) can help us to obtain the vertical distribution of the atmospheric CO2 concentration, which is important to the study of carbon sources and carbon sinks. We design a seeder injected pulsed laser system, working as the laser source of the CO2 DIAL. Unlike the other CO2 DIALs, our laser source is the result of difference frequency of two lasers at the wavelengths of 1064 nm and 634 nm, respectively. It should be pointed out that the high frequency (wavelength) accuracy and stability of the emission laser, especially the on-line one, are greatly required in the CO2 DIAL system. However, the mechanical properties of the dye laser (634 nm) and the application of laser difference frequency technique make the wavelength drift constantly; besides, the extremely unstable energy of the pulsed laser increases the difficulty in identifying and stabilizing the on-line wavelength. Hence, a fast and efficient frequency (wavelength) stabilization method is needed to achieve a stable emission wavelength. Aiming at the research gap of the high precision requirements of on-line laser for this kind of pulsed DIAL, we propose a frequency stabilization method based on matching algorithm. The basic idea is to utilize the saturable absorption of CO2 molecule, by measuring the differential residual-intensity after the laser passing through dual absorption cells to calculate the optical depth (OD) and obtain the so-called pseudo CO2 absorption spectrum, which can be used to identify the on-line laser accurately. Finally, based on the matching algorithm of one-dimensional image, treating the OD as the gray value in the image, we implement the OD matching as a most important part in the process of frequency stabilization, and determine the exact position of the real-time output laser in the measured pseudo absorption spectrum. Thus, when some errors happen to the monitored ODs, by continuously adjusting the wavelength of the laser, the proposed method can fulfill the wavelength adjustment and accomplish the continuous frequency stabilization for on-line laser. Experimental results show that the frequency stabilization algorithm based on OD matching can satisfy the requirements for pulsed on-line laser frequency stabilization, and the sum of squares of deviation method is the optimal one in this application, with a stabilization accuracy of 0.3 pm. Besides, the proposed method can also be introduced in other laser frequency stabilization.

A method for measuring the frequency stability of tunable laser is proposed by using confocal Fabry-Perot (F-P) interferometer. The F-P interferometer is used to get the output frequency of the laser as a reference, and the method eliminates the need of an independent optical source as a frequency reference. Using this technique, the frequency stability of the tunable external-cavity diode laser (ECDL) is measured to be 2.26×10−9 with an integration time of 20 ms.

We describe a compact, all fiber, frequency stabilized diode laser system at 2051 nm using CO2 gas-filled Kagome Hollow Core Fiber (HCF), capable of tuning continuously over four transitions in 12C16O2: R(24), R(26), R(28), and R(30). This laser system has been designed for use in future space-based atmospheric monitoring using differential absorption lidar (DIAL). The fully spliced Kagome HCF gas cell is filled to 2 kPa CO2 partial pressure and we compare the observed CO2 lineshape features with those calculated using HITRAN, to quantify the properties of the CO2-filled fiber cell. In this first demonstration of Kagome HCF used in a fully sealed gas cell configuration for spectroscopy at 2 µm, we characterize the frequency stability of the locked system by beat frequency comparison against a reference laser. Results are presented for the laser locked to the center of the 12C16O2 R(30) transition, with frequency stability of ∼40 kHz or better at 1 s, and a frequency reproducibility at the 0.4-MHz level over a period of > 1 month. For DIAL applications, we also demonstrate two methods of stabilizing the laser frequency ~3 GHz from this line. Furthermore, no pressure degradation was observed during the ~15-month period in which frequency stability measurements were acquired.

Frequency tuning and stability of Nd:YVO 4 in a dual coupled cavity

The single-step Rydberg excitation of cesium atoms requires a 319 nm ultraviolet laser with a narrow laser linewidth, high frequency stability, and high output power. To meet these requirements, in this work, we construct a high-power, single-frequency UV laser system at this wavelength. In this system, the frequency stabilization of the 1560.492 nm seed laser is critical to the performance of the ultraviolet laser. We employ nonlinear frequency conversion technology, the 1560.492 nm laser is frequency-doubled to 780.246 nm via a single pass through a PPLN crystal, and function integration is realized based on the modular parameter adjustment interface provided by the PyRPL software. Subsequently, the 1560.492 nm laser is stabilized to the D2 hyperfine transition line of Rb-87 atoms using polarization spectroscopy (PS) and radio-frequency-modulated saturation absorption spectroscopy (RF-SAS). A comparative study of these two techniques shows that RF-SAS achieves superior stabilization performance, with the residual frequency fluctuation of the frequency-doubled laser being 1.07 MHz over 30 min. According to frequency doubling theory, the actual residual frequency fluctuation of the 1560.492 nm fundamental-frequency laser can be calculated as 0.535 MHz. Compared with our earlier scheme that utilized an ultra-low-expansion (ULE) optical cavity as a frequency reference, the present scheme eliminates the long-term drift induced by environmental factors. In contrast to frequency stabilization relying on discrete instruments, this integrated scheme significantly reduces the cost, simplifies the system architecture, saves space, and greatly enhances the flexibility and controllability of the system. It therefore provides a reliable and cost-effective solution to ensure the portability and practicability of high-performance UV laser sources. This high-precision frequency stabilization scheme directly guarantees the performance of the 319 nm UV laser, suppressing its linewidth below 10 kHz. Thus, it fully meets the stringent laser linewidth and frequency stability requirements for the single-step Rydberg excitation of cesium atoms and provides a reliable light source foundation for subsequent precision spectroscopic measurements.

An electromagnetically induced transparency (EIT) signal is observed in a V-type energy level scheme in a cesium vapor cell at room temperature. The effects of frequency detuning and the intensity of the pump laser on the EIT signal have been investigated. The performance of the probe-diode laser system, which is frequency stabilized on the EIT signal by using electrical feedback, is explored. The first derivative of the EIT signal offers a steeper slope and better S/N ratio for laser frequency stabilization than that of the Doppler-free hyperfine and crossover resonances. A comparative study of the frequency stability of an external-cavity diode laser stabilized at EIT and at the crossover resonance is presented. The square root of the Allan variance (σ) vs. integration time (τ) plot shows about a tenfold improvement in the frequency stability of the EIT-locked laser (σ ∼ 2.043 × 10−13τ−1/2) over that of the crossover-locked laser under a short integration time (1–10 ms), whereas a twofold improvement is found under a long integration time (∼1 s).

We present a new method for accurate mid-infrared frequency measurements and stabilization to a near-infrared ultra-stable frequency reference, transmitted with a long-distance fibre link and continuously monitored against state-of-the-art atomic fountain clocks. We demonstrate the frequency stabilization of a mid-infrared laser with fractional stability better than 4×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-14</sup> at 1 s averaging. This new stabilization scheme gives us the ability to transfer frequency stability in the range of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-15</sup> or even better, currently accessible in the near-infrared or in the visible, to mid-infrared lasers in a wide frequency range.

We demonstrate frequency stabilization of DFB laser via modulation transfer spectroscopy, which avoids the interference of Doppler absorption spectrum. By modulating the frequency of laser beam(pump laser beam), the other laser beam(probe laser beam) overlap pump laser beam is modulated as the same frequency by nonlinear interaction. By demodulating the signal from detection beam, an error signal is generated through the low pass filter. The frequency is stabilized at the 0 point position of the MTS frequency discrimination curve by PID loop which shows frequency fluctuation is less than 149 kHz, and the relative frequency stability is improved by nearly two orders of magnitude.

Two active hydrogen masers designed to be operated as clocks and as contributors to the National Research Council of Canada (NRC) time scale have been constructed. Details of their design are presented together with some initial measurements of line-Q and frequency stability. A plot of maser power as a function of linewidth for maser H4 is shown. The experimental values of linewidth at the extremes of the oscillation curve correspond to line-Q values of 6.1*10/sup 9/ and 1.3*10/sup 9/, respectively. At a reasonable operating atom flux, the line-Q is 4*10/sup 9/, which is a factor of two better than that obtained for maser H1 at its normal operating flux. Some initial measurements of the frequency stability of the masers have also been made. >

We investigate the frequency offset in the frequency stabilization of a semiconductor laser based on a frequency-dithering technique. An analytical model is presented to describe the effects of the amplitude modulation and the phase delay between the amplitude and frequency modulation on the frequency stabilization. We also experimentally and analytically show that the frequency offset could be reduced by using an appropriate phase-sensitive detection.

We report on frequency stabilization of multiple wavelength lasers operating at 1389 and 1695 nm simultaneously on a broadband spectrum. These lasers are implemented in ytterbium optical lattice clock experiments, which need to have a narrow enough linewidth and maintain high long-term frequency stability. A 1560 nm femtosecond mode-locked laser with a narrow mode spacing of 250 MHz is used as a master laser, which is referenced to a local ultrastable optical cavity with the instability better than 1 × 10−15 at 1 s averaging time. Through the combination of erbium-doped fiber amplifier and high nonlinear fiber, the spectral width of the maser laser is broadened from 10 nm to more than 300 nm. The range of the broadened spectrum can cover 1389 and 1695 nm. Meanwhile, the spectral intensity at the corresponding wavelength can ensure that the signal-to-noise ratio of the beat signals between the two lasers and the broadened spectrum is about 30 dB at a resolution bandwidth (RBW) of 100 kHz. After phase locking the 1389 and 1695 nm lasers on the broadband spectrum, the residual linewidths are obtained to be about 0.8 Hz at 1 Hz RBW, and the stabilities are 3.5 × 10−16 and 4.7 × 10−16 at 1 s averaging time respectively, improving about six orders of magnitude. Our result can be conducive to obtaining the stabilized laser sources for the atomic optical clock, and will be of great significance for simplifying and miniaturizing the optical clock system.

Precise time and frequency synchronization measurements are critical for many advanced applications, including mobile communications, the internet of things (IoT), and low earth orbit (LEO) satellite internet. Real-time precise point positioning (RT-PPP) technology has become one of the dominant methods for real-time high-precision time-frequency transfer and synchronization. This study proposes an enhanced BDS RT-PPP time transfer and synchronization method utilizing bias modeling and develops a high-precision RT-PPP time synchronization terminal. After correcting inconsistent pseudorange biases, the RT-PPP time transfer precision reaches 0.19 ns for GPS and 0.16 ns for BDS, while the frequency stability over an averaging time of 1.5×104 s reaches 4.01×10-15. Compared with the conventional RT-PPP solution, the proposed bias-corrected method improves the time and frequency transfer performance of both GPS and BDS by approximately 30%. Furthermore, after clock information processing based on BDS RT-PPP, real-time pulse-per-second (PPS) physical signal synchronization with an average precision of 0.20 ns is achieved, and average frequency stability reaches 2.56×10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-15</sup> at 1 day averaging time in two time links. When combined with a low-cost oven-controlled crystal oscillator (OCXO), the proposed system demonstrates frequency stability comparable to that of commercial hydrogen masers at 1 day averaging time, providing a portable solution for modern time and frequency stability measurements.

We explored the rich satellite hole structures emerging as a result of spectral hole burning in cyanobacterial photosystem I (PSI) and demonstrated that hole burning properties of PSI, particularly at high resolution, are strongly affected by the oxidation state of the primary donor P700, as P700+ effectively quenches the excitations of the lowest-energy antenna states responsible for fluorescence. Obtaining better control of this variable will be crucial for high-resolution ensemble experiments on protein energy landscapes in PSI. The separate nonphotochemical spectral hole burning (NPHB) signatures of various red antenna states were obtained, allowing for additional constraints on excitonic structure-based calculations. Preliminary evidence is presented for an additional red state of PSI of T. elongatus peaked at 712.6 nm, distinct from previously reported C708 and C715 states and possibly involving chlorophyll B15. Excitation at wavelengths as long as 800 nm results in charge separation at cryogenic temperatures in PSI also in Synechocystis sp. PCC 6803. Both the "P700+ minus P700" holes and nonphotochemical spectral holes were subjected to thermocycling. The distribution of barriers manifesting in recovery of the "P700+ minus P700" signature contains two components in sample-dependent proportions, likely reflecting the percentages of FA and FB clusters being successfully prereduced before the optical experiment. The barrier distribution for the recovery of the lower-energy nonphotochemical spectral holes resembles those observed for other pigment-protein complexes, suggesting similar structural elements are responsible for NPHB. Higher-energy components exhibit evidence of "domino effects" such as shifts of certain bands persisting past the lower-energy hole recovery. Thus, conformational changes triggered by excitation of one pigment likely can affect multiple pigments in this tightly packed system.

Single frequency laser sources with high frequency stabilization serve as the backbone for the fields from advanced scientific experiments to industrial or commercial applications, thanks to the rapid development of laser frequency stabilization techniques based on optical or electrical feedback from an external reference. Despite the tremendous progress, it still remains an open question as to how to realize a laser source that can reach the high frequency stabilization and still remain sufficiently compact and portable for field use, with the increasing interest in applying these systems outside of the laboratory. Here, we describe a frequency stabilization DFB fiber laser with intracavity frequency reference mechanism, by utilizing the beat note of dual modes inside the laser cavity as the laser wavelength drift reference signal. As one of the best results for the laser frequency stabilization without the external frequency reference, the outcome of this work presents a way to achieve a stable laser source as an alternative to the laser-locked to absorption line or FP cavity design with a simpler, lower cost feature, offering promising prospects for versatile applications outside of the laboratory.