External Cavity Diode Laser Research Articles

Stabilizing Frequency of a Diode Laser to a Reference Transition of Molecular Iodine through Modulation Transfer Spectroscopy

We report the frequency stabilization of an external cavity diode laser (ECDL) to a reference molecular iodine (I2) transition at 13,531.18 cm−1 (739.03382 nm). Using the Modulation Transfer Spectroscopy (MTS) method for the highly sensitive detection of weak absorption signals, the Doppler-free absorption peaks of I2 corresponding to the hot band transition R(78) (1–11) are resolved. The ECDL’s frequency is stabilized with respect to one of the lines lying within the reference absorption band. For this, the iodine vapor cell is heated to 450 °C and the corresponding circularly polarized pump and probe beam powers are maintained at 10 mW and 1 mW, respectively, to avoid power broadening. The short (100 ms) and long-term (50 h) linewidths of the frequency stabilized laser are measured to be 0.75(3) MHz and 0.5(2) MHz, respectively, whereas the natural linewidth of the specific I2-transitions lie within a range of tens of MHz.

High‐Resolution Investigation of a Grating‐Stabilized Laser with a Fabry–Pérot Interferometer

A diode‐based external cavity diode laser (ECDL) is built in Littrow configuration and its modal behavior as well as its current and temperature‐induced wavelength shift is investigated. The ECDL is based on a laser diode (LD) in the blue spectral range with a straight ridge and low‐reflectivity front facet. A high‐resolution grating spectrometer (SPEX 1404 0.85 m double spectrometer) as well as a scanning Fabry–Pérot Interferometer (FPI, Burleigh RC‐110 with RG‐93 ramp generator) are used for spectral characterization. With the high resolution of the FPI, an upper bound of the ECDL linewidth of about 50 fm is found, which is 75 MHz in terms of frequency. The spectral shifts with temperature and with driving current are determined. The shift of 1.15 pm mA−1of the individual mode of the external cavity is caused by a change of the group refractive index and corresponding change of the effective cavity length. The shift of 2.7 pm mA−1of the spectral center of gravity can be explained by the faster shift of the internal modes. The single‐mode shift with temperature of 16.71 pm K−1is consistently measured both by the spectrometer and the FPI.

Low-drift-rate external cavity diode laser

We present the design, construction, and simulation of a simple, low-cost external cavity diode laser with a measured free-running frequency drift rate of 1.4(1) MHz/h at 852 nm. This performance is achieved in a compact aluminum structure held inside an airtight, temperature-controlled enclosure. The high thermal conductivity of the laser cavity and the stable temperature environment inside the enclosure minimize the time-varying, spatial temperature gradients across the laser cavity. We present thermal finite element method simulations, which quantify the effects of temperature gradients, and suggest that the drift rate is likely limited by the laser-diode and piezo-aging.

Revising the H2 16O line-shape parameters around 1.1 μm based on the speed-dependent Nelkin–Ghatak profile andthe Hartmann–Tran profile

Accurate spectroscopic data for H2 16O in the 1.1 μm region are particularly important for the study of Earth’s atmosphere. The pure water vapor molecular spectra were measured based on direct laser absorption spectroscopy using a narrow line-width external cavity diode laser combined with a high-precision Fabry–Pérot etalon. A total of 31 H2 16O transitions were studied for the first time by using the speed-dependent Nelkin–Ghatak profile and the Hartmann–Tran profile. From an accurate line-shape analysis, we obtained the line intensities and the self-broadening coefficients, and they are compared with the available data reported in the HITRAN 2016 database and the HITRAN 2020 database. Finally, we obtained information on the influence of Dicke narrowing, as well as the correlations between Dicke narrowing and speed dependence, and of speed-dependent effects.

Measurement of hyperfine constants and the isotope shift of rubidium 5P1/2 excited-state using saturated absorption spectroscopy

The Saturated Absorption Spectroscopy (SAS) was performed to measure the hyperfine constants of rubidium 5P1/2 excited state using an external-cavity diode laser (ECDL) operating at 795 nm. We report our measurements for the magnetic dipole coupling constants 120.79(29) and 407.75(57) for 85 Rb and 87 Rb respectively. Our measurement of the 85 Rb hyperfine constant shows a good agreement with previous results and our measurement of the 87 Rb hyperfine constant resolves the discrepancy of results that exists in the literature.

Cs microcell optical reference with frequency stability in the low 10-13 range at 1 s.

We describe a high-performance optical frequency reference based on dual-frequency sub-Doppler spectroscopy (DFSDS) using a Cs vapor microfabricated cell and an external-cavity diode laser at 895 nm. Measured against a reference optical signal extracted from a cavity-stabilized laser, the microcell-stabilized laser demonstrates an instability of 3 × 10-13 at 1 s, in agreement with a phase noise of +40 dBrad2/Hz at 1-Hz offset frequency, and below 5 × 10-14 at 102 s. The laser short-term stability limit is in good agreement with the intermodulation effect from the laser frequency noise. These results suggest that DFSDS is a valuable approach for the development of ultra-stable microcell-based optical standards.

Stable 671-nm external cavity diode laser with output power exceeding 150 mW suitable for laser cooling of lithium atoms.

We report the design and performance of a Littrow-type 671-nm external cavity diode laser (ECDL) that delivers output power greater than 150 mW and features enhanced passive stability. The main body of the ECDL is constructed using titanium to minimize temperature related frequency drifts. The laser diode is mounted in a cylindrical mount that allows vertical adjustments while maintaining thermal contact with the temperature stabilized baseplate. The wavelength tuning is achieved by horizontal displacement of the diffraction grating about an optimal pivot point. The compact design increases the robustness and passive stability of the ECDL and the stiff but lightweight diffraction grating-arm reduces the susceptibility to low-frequency mechanical vibrations. The linewidth of the ECDL is ∼360 kHz. We use the 671-nm ECDL, without any additional power amplification, for laser cooling and trapping of lithium atoms in a magneto-optical trap. This simple, low-cost ECDL design using off-the-shelf laser diodes without anti-reflection coating can also be adapted to other wavelengths.

External-cavity diode laser-based near-infrared broadband laser heterodyne radiometer for remote sensing of atmospheric CO2.

A near-infrared broadband (1500-1640 nm) laser heterodyne radiometer (LHR) with a tunable external-cavity diode laser as the local oscillator is developed and the relative transmittance, which represents the absolute relationship between the measured spectral signals and the atmospheric transmittance, is derived. High-resolution (0.0087 cm-1) LHR spectra in the spectral region of 6248.5-6256 cm-1 were recorded for the observation of atmospheric CO2. Combined with the relative transmittance, the preprocessed measured LHR spectra, the optimal estimation method, and the Python scripts for computational atmospheric spectroscopy, the column-averaged dry-air mixing ratio of CO2 of 409.09 ± 8 ppmv in Dunkirk, France on February 23, 2019, was retrieved, which is consistent with GOSAT and TCCON data. The near-infrared external-cavity LHR demonstrated in the present work has a high potential for use in developing a robust, broadband, unattended, and all-fiber LHR for spacecraft and ground-based atmospheric sensing that offers more channel selection for inversion.

Laser frequency stabilization in the 10 -14 range via optimized modulation transfer spectroscopy on the 87Rb D2 line.

We present a high-performance laser frequency stabilization method using modulation transfer spectroscopy (MTS) on the rubidium 87D2 transition line. A substantial improvement of the laser frequency stability was achieved by searching for the optimal diameter and intensity settings of the probe and pump beam. The frequency instability measured from the beat frequency of two locked external cavity diode lasers (ECDLs) reached a short-term stability of 4.5×10-14/τ and did not exceed 2 × 10-12 until 105 s, which is the best performance reported thus far with a D2 transition. The long-term stability is limited by the offset fluctuations of the baseline induced by the residual amplitude modulation (RAM), which can be further improved by reducing the current temperature variation of about 0.2 K by means of temperature stabilization or through a further reduction of the RAM.

Ambient Hydrocarbon Detection with an Ultra-Low-Loss Cavity Raman Analyzer.

The detection of ambient outdoor trace hydrocarbons was investigated with a multipass Raman analyzer. It relies on a multimode blue laser diode receiving optical feedback from a retroreflecting multipass optical cavity, effectively creating an external cavity diode laser within which spontaneous Raman scattering enhancement occurs. When implemented with ultra-low-loss mirrors, a more than 20-fold increase in signal-to-background ratio was obtained, enabling proximity detection of trace motor vehicle exhaust gases such as H2, CO, NO, CH4, C2H2, C2H4, and C2H6. In a 10-min-long measurement at double atmospheric pressure, the limits of detection obtained were near or below 100 ppb for most analytes.

Continuously and widely tunable frequency-stabilized laser based on an optical frequency comb.

Continuously and widely tunable lasers, actively stabilized on a frequency reference, are broadly employed in atomic, molecular, and optical (AMO) physics. The frequency-stabilized optical frequency comb (OFC) provides a novel optical frequency reference, with a broadband spectrum that meets the requirement of laser frequency stabilization. Therefore, we demonstrate a frequency-stabilized and precisely tunable laser system based on it. In this scheme, the laser frequency locked to the OFC is driven to jump over the ambiguity zones, which blocks the wide tuning of the locked laser, and tuned until the mode hopping happens with the always-activated feedback loop. Meanwhile, we compensate the gap of the frequency jump with a synchronized acoustic optical modulator to ensure the continuity. This scheme is applied to an external cavity diode laser (ECDL), and we achieve tuning at a rate of about 7 GHz/s, with some readily available commercial electronics. Furthermore, we tune the frequency-stabilized laser only with the feedback of diode current, and its average tuning speed can exceed 100 GHz/s. Due to the resource-efficient configuration and the simplicity of completion, this scheme can be referenced and can find wide applications in AMO experiments.

Measurement of self-broadening coefficients and line intensities of ν1 + ν3 + ν4 ‒ ν4 band of acetylene in 1.5 μm spectral region: External cavity diode laser based absorption study

Acetylene is considered to be an important molecule for spectroscopic research due to its role in environmental chemistry, astrophysics, biochemistry and photochemistry of various planets. The high resolution spectroscopic studies of acetylene (12C2H2) in the 6474–6515 cm−1 spectral range have been performed using in-house developed external-cavity diode laser absorption spectroscopy technique. The study is focused on experimental measurement of self-broadening coefficients (γself) and line intensities of 27 l-doublet ro-vibrational lines belonging to the P-branch transition of ν1 + ν3 + ν4 ‒ ν4 hot band lying in the 6474–6515 cm−1 spectral range. The γself and line intensity have been retrieved by modelling of each experimentally observed rotational line with standard Voigt line profile using custom written python based computer program. The measured γself and line intensities were compared with the values present in HITRAN2020 database, showing mean difference of about -1.9% and 2.48%, respectively. In contrast to the line intensity, where rotational and symmetry dependency are readily visible, γself exhibits only rotational dependency. The l-type doubling constants of both ν1 + ν3 + ν4 and ν4 states of ν1 + ν3 + ν4 ‒ ν4 hot band have also been obtained. Detailed understanding of these molecular parameters will play an important role in modelling of planetary atmospheres.

1.7 THz tuning range pivot-point-independent mode-hop-free external cavity diode laser.

A novel mode-hop-free (MHF) tunable external cavity diode laser (ECDL) is demonstrated without mechanical pivot-point tuning. By corotating a periscope with an etalon and a narrow band pass (NBP) filter inside an external cavity, the cavity single longitudinal mode selection can be maintained, and continuous tuning can be achieved with optimal synchronization. A MHF continuous tuning range of 1.7 THz has been achieved with a semiconductor gain chip at the wavelength of 652 ± 2 nm experimentally, and the theoretical tuning range can reach over 4.8 THz. The laser linewidth is estimated to be less than 1 MHz (FWHM) by a scanning Fabry-Perot (F-P) and a Michelson interferometer. 1 mW output power with variation of less than 10% in the tuning region of roughly 1.7 THz has been demonstrated.

Laser system of cold atom optical clock in China Space Station

The world's first space optical clock (SOC) developed in China, which is composed of five subsystems, i.e. an optical unit, a physics unit, an electronic control unit, a space optical frequency comb, and an ultrastable laser, was successfully launched with the Mengtian space laboratory on October 31, 2022, and entered into the China Space Station (CSS). Compact and stable laser is a key element for the operation of the SOC. The optical unit consists of 5 lasers with wavelengths of 461, 679, 689, 707 and 813 nm, respectively. With a synchronous-tuning-like scheme, high-quality external cavity diode lasers (ECDLs) are developed as the seeds. The linewidths of the lasers are all reduced to approximately 100 kHz, and their tuning ranges, free from mode hopping, are capable of reaching 20 GHz, satisfying the requirements for the SOC. With careful mechanical and thermal design, the stability of the laser against vibration and temperature fluctuation is sufficiently promoted to confront the challenge of rocket launching. While the power from the ECDL is sufficient for 679-nm repump laser and 707-nm repump laser, additional injection lock is utilized for the 461-nm laser and 689-nm laser to amplify the power of the seeds to more than 600 mW, so that effective first and second stage Doppler cooling can be achieved. To generate an optical lattice with deep enough potential well, over 800-mW 813-nm lasers are required. Therefore, a semiconductor tapered amplifier is adopted to amplify the seed to more than 2 W, so as to cope with various losses of the coupling optics. The wavelengths and output power values of the 5 lasers are monitored and feedback is controlled by the electronic control unit. All the modules are designed and prepared as orbital replaceable units, which can be easily replaced by astronauts in case failure occurs. Now the lasers are all turned on and operate normally in CSS. More data of the SOC will be obtained in the near future. At present stage, according to our evaluation, the continuous operation time of the SOC is limited by the injection locked lasers, which are relatively vulnerable to mode hopping. Hopefully, this problem can be solved by improving the laser diode preparing technology, or developing fiber lasers with compact frequency conversion modules.

12CO2 transition frequencies with kHz-accuracy by saturation spectroscopy in the 1.99-2.09 μm region.

Saturation spectroscopy has been used to determine the absolute frequencies of 107 ro-vibrational transitions of the two strongest 12CO2 bands of the 2 μm region. The considered 20012-00001 and 20013-00001 bands are of importance for the CO2 monitoring in our atmosphere. Lamb dips were measured using a cavity ring-down spectrometer linked to an optical frequency comb referenced to a GPS-disciplined Rb oscillator or to an ultra-stable optical frequency. The comb-coherence transfer (CCT) technique was applied to obtain a RF tunable narrow-line comb-disciplined laser source using an external cavity diode laser and a simple electro-optic modulator. This setup allows obtaining transition frequency measurements with kHz-level accuracy. The resulting accurate values of the energy levels of the 20012 and 20013 vibrational states are reproduced with a (1σ)-rms of about 1 kHz using the standard polynomial model. The two upper vibrational states appear thus to be highly isolated except for a local perturbation of the 20012 state leading to an energy shift of 15 kHz at J = 43. A recommended list of 145 transition frequencies with kHz accuracy is obtained providing secondary frequency standards across the 1.99-2.09 μm range. The reported frequencies will be valuable to constrain the zero-pressure frequencies of the considered transitions in 12CO2 retrieval from atmospheric spectra.

Practical Talbot wavemeter

A wavemeter using the near-field Talbot diffraction is simplified. High accuracy wavelength measurement can be obtained from each spacing between two adjacent maximum intensities of the periodicity along multiples of the Talbot distance. Our experimental results are confirmed by our calculations. In contrast to the previous works, we use a diffraction grating with sufficiently large grating period. Therefore, the setup is practical and the pixel size of camera used to measure the interference pattern can be large. Moreover, the obtained Talbot patterns are sharp without using a post-image processing. According to our recent setup, we use the grating with a period of 12.5 μm. With visible light lasers, the Talbot distances are in the range of a few hundred of micrometers. These distances are much larger than a pixel size of normal camera. An external cavity diode laser with rubidium saturated absorption spectroscopy is used to calibrate our setup. High accuracy at 1 pm can be achieved.

Design and fabrication of an optical film for fiber bragg grating external cavity diode lasers

The predecessor of China Optics was China Optics and Applied Optics Digest, which was founded in 1985. At that time, it was the only retrieval journal in the field of optics in China. At the end of 2008, China Optics and Applied Optics Digest was renamed

Underwater Frequency Modulated Continuous Wave Laser Ranging Based on Blue External Cavity Diode Laser

Underwater Frequency Modulated Continuous Wave Laser Ranging Based on Blue External Cavity Diode Laser

Comb-assisted mercury spectroscopy in the deep-ultraviolet: towards the development of a new primary thermometer

We report on the development of a new primary thermometer based upon high-precision spectroscopy of mercury vapors in the deep-ultraviolet region for the practical realization of the new kelvin. The line profile of the (6s 2)1 S 0 → (6s6p)3 P 1 intercombination transition of the 200Hg bosonic isotope is observed with a high spectral fidelity using a coherent radiation source at 253.7 nm. This latter consists of a near-IR external cavity diode laser followed by a double-stage second-harmonic generation apparatus. Metrology grade UV spectroscopy is demonstrated by locking the diode laser to a self-referenced optical frequency comb synthesizer.

Near-infrared detection and line intensity measurements of H2S with ECDL source based off-axis integrated cavity output spectroscopy

Hydrogen sulphide (H2S) is an extremely hazardous gas which leads to poisoning at higher concentration. Here, we demonstrate the trace detection and line intensity measurement of H2S by means of efficient and indigenously developed off–axis integrated cavity output spectroscopy (OA-ICOS) technique coupled with an external cavity diode laser operating in near-infrared spectral region. Trace detection of H2S was made with OA-ICOS absorbance spectra of ro-vibrational line (51, 5 → 61, 6 & 50, 5 → 60, 6) centered at ∼ 6344 cm−1 of ν1 + ν2 + ν3 band. A ∼ 600 ppbv limit of detection was achieved for an averaging time of 350 s with Allan deviation analysis. Additionally, line intensities of several degenerate R branch transitions of ν1 + ν2 + ν3 band including 51, 5 → 61, 6 & 50, 5 → 60, 6 transition was experimentally measured with OA-ICOS. The study will be valuable in aiding the spectroscopic databases of H2S and have potential application in extracting molecular concentrations in the planet’s atmospheres.