Frequency-modulated Laser Diode Research Articles

High precision displacement measuring interferometer based on the active modulation index control method

This study proposes a simple, high-precision, and wide-measurement-range displacement measuring interferometer. A frequency-modulated laser diode (LD) source at 30 MHz was employed for the interferometer. The interference signal comprised a series of harmonics of the modulation frequency. Normally, the signal-to-noise ratio (SNR) of harmonics changes rapidly because of the unbalanced length between the two arms of the interferometer; hence, the measurement accuracy and range were limited. In this study, the SNR of the harmonics was improved by controlling the modulation excursion of the LD. Moreover, all harmonics with high SNR in a specific measurement range can be detected simultaneously using lock-in amplifiers (LIAs). Therefore, the displacement was determined using the pair of harmonics with the best SNR from the detected harmonics. To verify the measurement accuracy, the proposed interferometer was utilised to measure the nanoscale displacement induced by a piezo actuator (PZT) with sinusoidal and triangular shapes.

Измерение расстояния по максимальной частоте интерференционного сигнала при гармонической девиации длины волны лазерного автодина

A method is proposed for measuring absolute distances from the low-frequency spectrum of the interference signal of a frequency-modulated laser diode. The method of modulation of the self-mixing laser signal according to the harmonic law is used, in the spectrum of which a wide range of frequency components is observed. The connection between the maximum frequency of change of the interference signal and the absolute distance to the reflector is shown. A linear dependence of the frequency corresponding to the inflection region of the decay envelope of the interference signal spectrum on the distance is shown, which can be used to implement a non-contact distance measurement method with harmonic modulation of the self-mixing laser supply current.

Measurement of distance by the maximum frequency of the interference signal with harmonic deviation of the wavelength of the self-mixing laser

A method is proposed for measuring absolute distances from the low-frequency spectrum of the interference signal of a frequency-modulated laser diode. The method of modulation of the self-mixing laser signal according to the harmonic law is used, in the spectrum of which a wide range of frequency components is observed. The connection between the maximum frequency of change of the interference signal and the absolute distance to the reflector is shown. A linear dependence of the frequency corresponding to the inflection region of the decay envelope of the interference signal spectrum on the distance is shown, which can be used to implement a non-contact distance measurement method with harmonic modulation of the self-mixing laser supply current. Keywords: laser interferometry, self-mixing laser, semiconductor laser, laser radiation modulation, distance measurement, signal spectral analysis.

The 1.66 μm spectrum of the ethynyl radical, CCH

Frequency-modulated diode laser transient absorption spectra of the ethynyl radical have been recorded at wavelengths close to 1.66 μm. The observed spectrum includes strong, regular, line patterns. The two main bands observed originate in the ground X̃2Σ+ state and its first excited bending vibrational level of 2Π symmetry. The upper states, of 2Σ+ symmetry at 6055.6 cm−1 and 2Π symmetry at 6413.5 cm−1, respectively, had not previously been observed and the data were analyzed in terms of an effective Hamiltonian representing their rotational and fine structure levels to derive parameters which can be used to calculate rotational levels up to J = 37/2 for the 2Π level and J = 29/2 for the 2Σ one. Additionally, a weaker series of lines have been assigned to absorption from the second excited bending, (0 2 0), level of 2Σ symmetry, to a previously observed state of 2Π symmetry near 6819 cm−1. These strong absorption bands at convenient near-IR laser wavelengths will be useful for monitoring CCH radicals in chemical systems.

Frequency-modulated diode laser frequency combs at 2 μm wavelength

Chip-scale electrically pumped optical frequency combs (OFCs) are expected to play a fundamental role in applications ranging from telecommunications to optical sensing. To date, however, the availability of such sources around 2 μm has been scarce. Here, we present a frequency-modulated OFC operating around 2060 nm of wavelength exploiting the inherent gain nonlinearity of single-section GaSb-based quantum well diode lasers. A 2 mm long device operating as a self-starting comb outputs 50 mW of optical power over more than 10 nm of bandwidth while consuming <1 W of electrical power. Using the shifted-wave interference Fourier transform spectroscopy technique, we characterize the generated frequency-modulated waveform and demonstrate a linearly chirped intermodal phase relationship among the entire emission optical bandwidth. Furthermore, by compensating for the linear chirp using a single-mode optical fiber with opposite dispersion, 6 ps long optical pulses are generated. The frequency stability of the devices with ∼19.3 GHz repetition rates allows us to perform mode-resolved free-running dual-comb spectroscopy. All rights reserved.

A ppb level sensitive sensor for atmospheric methane detection

A high sensitivity sensor, combining a multipass cell and wavelength modulation spectroscopy in the near infrared spectral region was designed and implemented for trace gas detection. The effective length of the multipass cell was about 290 meters. The developed spectroscopic technique demonstrates an improved sensitivity of methane in ambient air and a relatively short detection time compared to previously reported sensors. Home-built electronics and software were employed for diode laser frequency modulation, signal lock-in detection and processing. A dual beam scheme and a balanced photo-detector were implemented to suppress the intensity modulation and noise for better detection sensitivity. The performance of the sensor was evaluated in a series of measurements ranging from three hours to two days. The average methane concentration measured in ambient air was 2.01ppm with a relative error of±2.5%. With Allan deviation analysis, it was found that the methane detection limit of 1.2ppb was achieved in 650s. The developed sensor is compact and portable, and thus it is well suited for field measurements of methane and other trace gases.

Concurrent measurement method of spindle radial, axial and angular motions using concentric circle grating and phase modulation interferometers

This paper describes a concurrent measurement of spindle radial, axial and angular motions using concentric circle grating and phase modulation interferometers. In the measurement, a concentric circle grating with fine pitch is the reference artifact. A frequency modulated laser diode is used for the light source of the phase modulation interferometers. The phase modulation interferometers, which consist of three Michelson and three grating interferometers, measure three radial and three axial displacements of the grating. From these six measured displacements, radial, axial and angular motions of a spindle can be determined concurrently. In the paper, a measurement principle, a measurement instrument and experimental results are discussed. Concurrent measurements of spindle radial, axial and angular motions were successfully attained.

Coherence function control of a multi-mode laser diode by the frequency modulation and its low coherence interferometric application

In this investigation, a frequency-modulated multi-mode laser diode (LD) is introduced as a light source of low coherence scanning interferometry (LCSI) and its applicability is verified with experiments. The frequency modulation of a multi-mode LD can improve the contrast of the coherence peak corresponding to zero OPD and overcome the ambiguity to find the exact coherence peak appeared in frequency modulated single-mode LD or a multi-mode LD. As the experimental results, the coherence function of a multi-mode LD was changed with the frequency modulation and the expected coherence peak was obviously determined. As the result, the coherence length was 200μm. Also, the possibility to measure surface profile of a sample was experimentally confirmed with step height samples. Although the proposed technique cannot be guaranteed for high accuracy in the lack of the position accuracy of the scanning stage, it is suitable for the demand of spatially coherent measurements and high speed measurements adopting the sub-sampling technique because of its low cost, high available power and better spatial coherence than other white light sources.

Spindle Error Motion Measurement Using Concentric Circle Grating and Phase Modulation Interferometers

In the conventional methods of measuring radial, axial, and angular motions of a spindle concurrently, complicated reference artifacts with relative large volume, e.g., two balls linked to a cylinder, are required. A simple and small artifact is favorable from the viewpoint of accurate measurement. This paper describes a concurrent measurement of radial, axial, and angular spindle motions using concentric circle grating and phase modulation interferometers. In the measurement, concentric circle grating with fine pitch is installed on top of the spindle of interest. The grating is a reference artifact in the method. Three optical sensors are fixed over the concentric circle grating, to observe its proper positions. The optical sensor consists of a frequency modulated laser diode as a light source as well as two interferometers. One interferometer observes an interference fringe between reflected light from a fixedmirror and 0-th order diffraction light from the grating to measure the axial motion. Another interferometer observes an interference fringe between ± 2nd order diffraction lights from the grating tomeasure the radialmotion. Using three optical sensors, three axial displacements, and three radial displacements of the proper observed position of the grating can be measured. From these measured displacements, radial, axial, and angular motions of the spindle can be calculated concurrently. In this paper, a measurement instrument, a novel fringe interpolation technique using sinusoidal phase modulation, and experimental results are discussed.

Assessment of photon migration in scattering media using heterodyning techniques with a frequency modulated diode laser

A novel technique for studying photon propagation in scattering media is proposed and demonstrated, as is believed, for the first time. Photons propagating through the medium, from a frequency-ramped single-mode diode laser, meet a reference beam from the same source, at a common detector, and beat frequencies corresponding to various temporal delays are observed by heterodyne techniques. Fourier transformation directly yields the temporal dispersion curve. Proof-of-principle experiments on polystyrene foam and a tissue phantom suggest, that the new method, when fully developed, may favorably compete with the more complex time-correlated single-photon counting (TCSPC) and the phase-shift methods, now much employed.

Self-mixing frequency-modulated laser interferometry

Methods of laser interferometry based on the optical feedback and frequency-modulated laser diode effects are described. An interference signal is analyzed for problems concerned with range finding, displacement estimation, and measurement of velocities and vibration parameters. Relationships between the object movement characteristics and the interference signal parameters are found. Advantages of the new trend in interferometry are an extremely high sensitivity to scattered light, the possibility of estimating the displacement direction and obtaining information from small object surface areas, simple design, and low cost of the interferometers.

Methyl Iodide Photodissociation at 193 nm: The I(2P1/2) Quantum Yield

Methyl iodide photolysis at 193 nm has been studied through probing the I((2)P(1/2)-(2)P(3/2)) transition in the atomic iodine photofragment using diode laser spectroscopy. The I((2)P(1/2)) quantum yield has been determined through two different diode laser techniques and then compared. Frequency-modulated diode laser based absorption spectroscopy was used to extract nascent Doppler lineshapes from which an I((2)P(1/2)) quantum yield of unity is inferred. However when diode laser gain/absorption measurements were made, an I((2)P(1/2)) quantum yield of 0.68 ± 0.04 was found. The reason for this discrepancy is shown to lie in the diode laser gain/absorption method. Molecular iodine is found to be formed during the experiment via atomic iodine recombination and then in turn dissociates to produce both I((2)P(1/2)) and I((2)P(3/2)), thus distorting the returned quantum yield. This conclusion is supported both by the reduction of the I((2)P(1/2)) quantum yield with number of photolysis laser shots when measured using this technique and by the presence of fluoresence which is shown to have excited-state lifetimes and quenching rates that are consistent with those previously measured for the D and D' states of molecular iodine.

All-Optical Magnetometer Based on Resonant Excitation of Rubidium Atoms by Frequency Modulated Diode Laser Light

Magnetic field and its gradient measurements based on coherent population trapping resonances at the D1 line of Rb are performed by means of coupling two ground-state Zeeman sublevels belonging to different hyperfine levels to a common excited state. Two coherent laser fields with frequency difference of 6.8 GHz are used. They are produced by direct current modulation of a diode laser. The resonance splitting and broadening caused by the applied magnetic field is measured by scanning the laser modulation frequency around the ground-state hyperfine frequency.

Fringe counting method for synthetic phase with frequency-modulated laser diodes

Fringe counting method with laser diodes (LDs) for displacement measurement has been constructed. Two LDs are frequency modulated by mutually inverted sawtooth currents on an unbalanced two-beam interferometer. The mutually inverted sawtooth-current modulation of LDs produces interference fringe signals with opposite signs for respective wavelengths. The two fringe signals are fed to an electronic mixer to produce a synthetic fringe signal with a reduced sensitivity to the synthetic wavelength. Synthetic fringe pulses derived from the synthetic fringe signal make a fringe counting system possible for faster movement of the tested mirror.

266 nm photolysis of CF3I and C2F5I studied by diode laser gain FM spectroscopy

Frequency modulated diode laser based absorption at 1.315 microm has been used to measure the Doppler lineshapes of the I((2)P(1/2)-(2)P(3/2)) transition in atomic iodine produced from the 266 nm photolysis of both CF(3)I and C(2)F(5)I. Wavelength resolved laser gain is seen following photolysis as excited iodine atoms ((2)P(1/2)) are produced with a quantum yield close to unity from photolysis of both parent molecules. Time resolved measurements were made and the nascent speed distribution and translational anisotropy parameter, beta were determined. Mean atomic speeds of 800 and 850 ms(-1), which correspond to 83 and 68% of the maximum possible kinetic energy release into the iodine photofragment, were determined for photolysis of CF(3)I and C(2)F(5)I, respectively. The nascent translational anisotropy parameter was found to be beta = 1.77 +/- 0.05 for CF(3)I and beta = 1.69 +/- 0.05 for C(2)F(5)I. These values are explicable in terms of parent rotational motion and non-adiabatic processes in the exit channel.

Voice transmission via frequency-modulated diode laser light

We have built a tabletop frequency modulation (FM) communication system that uses a laser field as the carrier. Voice information is encoded on the laser beam using an acousto-optic modulator, and it is extracted by a Michelson-type interferometer and a FM radio. This system can easily be converted to an intensity-modulated communication system. We discuss similar systems based on phase or polarization modulation using an electro-optic modulator.

Number density and temperature measurements obtained using sensitive diode laser spectroscopy in an argon plasma

The absolute concentration and translational temperature of the 2p10 and 2p7 excited states of argon have been measured in an inductively coupled plasma chamber under a variety of operating conditions using both calibrated diode laser frequency modulation spectroscopy and cavity enhanced absorption spectroscopy. Accurate lineshape analysis of frequency modulation signals has been employed to extract the desired information, and is corroborated by cavity enhanced measurements. Temperatures are found to vary linearly with pressure from ∼400 K at 20 mTorr (2.7 Pa) to ∼510 K at 90 mTorr (12 Pa) in a 200 W discharge while concentrations peak at 3.25×108 cm-3 at 30 mTorr (4 Pa) (also in a 200 W discharge). The uncertainty in the recovered temperature is 7%, dominated by uncertainties in the calibration of the frequency scale.

Birefringent fibre frequency-modulated continuous-wave Sagnac gyroscope

A birefringent fibre Sagnac gyroscope is described, which is based on the principle of optical frequency-modulated continuous-wave interference and primarily consists of a single-longitudinal-mode frequency-modulated laser diode and a birefringent fibre coil. The rotation velocity of the fibre coil can be determined simultaneously by detecting the phase shift of the beat signal produced by two counter propagating laser beams in the birefringent fibre coil. A resolution of 0.02°/s with a 100 m fibre coil has been observed. The advantages and limitation of the gyroscope are discussed.

Nonlinear dynamics of a laser diode with optical feedback systems subject to modulation

The nonlinear dynamic behavior of a direct frequency-modulated diode laser with strong optical feedback is examined and compared to a laser diode subject to electro-optically modulated, strong optical feedback. Direct modulation is achieved by sinusoidal modulation of the diode laser injection current. Electro-optic modulation is achieved by applying a sinusoidal voltage to an intracavity phase modulating element. The output state (characterized by the output power versus time, the intensity noise spectrum and the optical frequency spectrum) for both types of modulation is dependent on the ratio of the modulation frequency to the external cavity resonant frequency, and the modulation power. A number of distinct states are observed: conventional amplitude modulation (with FM spectra); multimode, low-noise amplitude modulation; multimode, high-noise amplitude modulation; periodic limit-cycle operation; quasi-periodicity; chaos; low-frequency fluctuations; and mode-locking. There are significant differences between the direct and electro-optic frequency-modulation cases. The onset of the dynamic instability is characterized as a noisy period-one oscillation for direct modulation and a low-frequency fluctuation for intracavity electro-optic modulation. Phase portraits produced experimentally with the use of a digital phosphor oscilloscope are shown to agree well with those constructed from output power versus time data. This represents an experimental method for examining the dynamics phase portraits in real-time.

Time-average holographic interferometry with a sinusoidally modulated laser diode

Laser diodes have the features of a single-mode operation and a frequency tunability. Holographic interferometry is described for investigating the vibrating amplitude and phase with a frequency-modulated laser diode. Sinusoidal wavelength change by varying laser injection current produces the sinusoidal phase modulation of relative phase difference between the reference and vibrating object paths. Holograms are made by time-average exposure of three-type recording modes with a laser diode. The phase of vibrating object is extracted from the measurements of Bessel-type fringe irradiances in three kinds of time-average holographic reconstruction. Experimental results of phase and amplitude for a vibrating cantilever object are shown.