Current-modulated Laser Research Articles

Simultaneous detection of CH4 and CO2 through dual modulation off-axis integrated cavity output spectroscopy

This study established a novel method for the simultaneous detection of two-component gases. Radio frequency (RF) white noise disturbance laser current and wavelength modulation were simultaneously used to improve the off-axis integrated cavity output spectroscopy technique, and a high-precision dual modulation OA-ICOS (RF-WM-OA-ICOS) system was established. The two laser beams were coupled into one laser beam that was applied incident to the cavity of RF-WM-OA-ICOS system. The second harmonic signals of CH4 and CO2 gas simultaneously appeared in the rising or falling edge of a triangular wave. This method was used to measure CH4 and CO2 with different concentrations. The results indicated that the proposed system has high stability and can accurately and simultaneously measure the concentrations of CH4 and CO2, with an optimal integration time of 220 s. The minimum detection limit was 10 ppb for CH4 and 1.5 ppm for CO2. The corresponding noise equivalent absorption sensitivity values were calculated as 2.67 × 10−13 cm−1⋅Hz−1/2 and 5.18 × 10−11 cm−1⋅Hz−1/2, respectively. The proposed dual-component gas simultaneous detection method can also be used for high-precision simultaneous detection of other gases. Therefore, this study may serve as a reference for developing portable multicomponent gas analyzers.

Excited oxygen kinetics at electronvolt temperatures via 5-MHz RF-diplexed laser absorption spectroscopy.

A multi-MHz laser absorption sensor at 777.2nm (12,863c m -1) is developed for simultaneous sensing of (1)O(5 S 0) number density, (2)electron number density, and (3)translational temperature at conditions relevant to high-speed entry conditions and molecular dissociation. This sensor leverages a bias tee circuit with a distributed feedback diode laser and an optimization of the laser current modulation waveform to enable temporal resolution of sub-microsecond kinetics at electronvolt temperatures. In shock-heated O 2, the precision of the temperature measurement is tested at 5MHz and is found to be within ±5% from 6000 to 12,000K at pressures from 0.1 to 1atm. The present sensor is also demonstrated in a CO:Ar mixture, in parallel with a diagnostic for CO rovibrational temperature, providing an additional validation across 7500-9700K during molecular dissociation. A demonstration of the electron number density measurement near 11,000K is performed and compared to a simplified model of ionization. Finally, as an illustration of the utility of this high-speed diagnostic, the measurement of the heavy particle excitation rate of O(5 S 0) is extended beyond the temperatures available in the literature and is found to be well represented by k(3 P→5 S 0)=2.7×10-14 T 0.5 exp⁡(-1.428×104/T)c m 3⋅s -1 from 5400 to 12,200K.

Resonant-Opto-Thermomechanical Oscillator (ROTMO): A Low-Power, Large Displacement, High-Frequency Optically Driven Microactuator.

A light-driven micromechanical oscillator is presented, which can be operated by a low optical power (in the mW, or even the µW range), can produce large mechanical displacements (>5-100µm), and can be designed to operate at frequencies from sub-kHz up to more than 200kHz. The actuation of the oscillator is achieved by an asymmetrically metal-coated optical microwire configured into a silica micromechanical oscillator. The metalized optical microwire confines and absorbs the light strongly over a short distance, which results in a controlled optical power conversion into heat, and, consequently, into mechanical actuation through the temperature rise and the difference in thermal expansions of the silica microwire and the asymmetrically applied metal layer. Mechanical displacements are amplified further by the resonance operation of the oscillator, which is driven by a low-power, harmonic optical excitation signal generated by a current-modulated laser diode. Proper selection of the micromechanical oscillator's geometrical configuration and materials allows for a high-frequency operation at large mechanical displacements of the oscillator, while relying on low excitation optical power. The presented concept of a fully optically driven micromechanical oscillator may, thus, present a basis for realization of new classes of actuated micro-opto-mechanical Systems and similar photonics microdevices.

High-Resolution and Large-Dynamic-Range π-Phase-Shifted FBG Sensor by a Current Modulated Diode Laser

We present a high-resolution and large dynamic range π-phase-shifted fiber Bragg grating (π-PSFBG) temperature sensor. A current modulated laser diode with a linearized optical frequency sweep is used to interrogate the sensor. The sensor is comprised of a π-PSFBG sensing head and an HCN gas cell. The gas cell functions as an absolute frequency reference to compensate for the laser frequency drift. The reflection spectrum is sent to a lock-in amplifier to generate the error signal, proportional to the first order derivative of the spectrum. The center frequency difference that carries sensing information between the π-PSFBG and HCN gas cell is calculated by cross-correlation of their error signals. A sinusoidal modulation enables to move the data processing from low-frequency stage to high-frequency stage, by phase-sensitive detection and low-pass filtering, the interferences caused by laser intensity noises and electronic devices’ noises are significantly reduced. In the experiment, a frequency resolution of 3.696 MHz with a 55 GHz mode-hop-free tuning range, is achieved, corresponding to a temperature resolution of 2.6 mK with a dynamic range from 5 to 40 °C.

Light shifts in the rubidium CPT atomic clock with laser current modulation at 3.4 and 6.8 GHz

In the paper presents results of an experimental comparison of the CPT parameters of the resonance at the D1 line in 87Rb when the laser current is modulated at frequencies 3.4 GHz and 6.8 GHz. Resonances are investigated for slope to noise ratio (Q-factor) and shifts from the microwave power. The reproducibility of the parameters of VCSEL lasers required to obtain long-term stability of an atomic clock is considered. The obtained CPT instabilities of the atomic clock for 1 second were 1.2·10−11 for the case of 3.4 GHz and 4·10−12 for 6.8 GHz.

Mode Locking of Dye Laser Pumped by Current-Modulated Green Laser Diodes

We report the mode locking regime of dye laser pumped by 400 ns laser diode pulses. Three green laser diodes operating at 513 nm are used as the pump source. We obtain subnanosecond pulse trains using periodic 1 ns pulse modulation of the power-supply current through the pump diodes. The addition of a saturable absorber results in both a decrease in the pulse duration to 0.2 ns and a good reproducibility of the output radiation..

Small-Signal Equivalent Circuit Model of Photonic Crystal Fano Laser

We develop a new small-signal equivalent circuit model for photonic crystal Fano laser based on standard linearized equations. The results are compared with those obtained by the numerical simulation approach. This novel circuit model being a cost-effective fast tool is advantageous over solving the rate equations for designing and extending the photonic crystal Fano laser. Using this circuit model, we have investigated the laser current modulation via the conventional method and via modulating the Fano mirror. The results show that the bandwidths of the amplitude modulation of the laser through-port and the frequency modulation of its cross port are both in the range of THz. It is also shown that the larger the nanocavity detuning, the greater the relaxation oscillation frequency, increasing the possibility of widening the laser bandwidth, through which the modulation efficiency may decrease. Moreover, we show that the longer the laser active region, the narrower the modulation response bandwidth. The results show that the circuit model accurately explains the photonic crystal Fano laser small-signal modulation characteristics.

Generation of Tunable and Ultra-Broadband Microwave Frequency Combs Based on a Semiconductor Laser Subject to Pulse Injection From a Current Modulated Laser

We have experimentally demonstrated and characterized the generation of tunable and ultra-broadband microwave frequency combs (MFCs) based on a slave laser (SL) subject to regular pulse injection from a current modulated master laser (ML). Under modulation frequency fm = 1.2 GHz and modulation power Pm = 22 dBm, the current modulated ML is driven into a regular pulse state, and a seed MFC with 14.4 GHz bandwidth within a ±5 dB amplitude variation can be obtained. Such a seed MFC is then injected into the SL for producing final MFC with higher performances. For a fixed detuning frequency fi = 0 GHz and optimized injection power Pi = 2060 μ W, the bandwidth of the final MFC is increased to 33.6 GHz, and the single-sideband phase noises of all comb lines of the MFC within the bandwidth can be decreased to below –90.9 dBc/Hz@10 kHz. Additionally, the influences of Pi and fm on the MFC bandwidth are also analyzed. The experimental results are in agreement with numerical simulations to a certain extent.

Wavelength-scanning coherent OTDR for dynamic high strain resolution sensing.

Distributed vibration sensing in optical fibers opened entirely new opportunities and penetrated various sectors from security to seismic monitoring. Here, we demonstrate a most simple and robust approach for dynamic strain measurement using wavelength-scanning coherent optical time domain reflectometry (C-OTDR). Our method is based on laser current modulation and Rayleigh backscatter shift correlation. As opposed to common single-wavelength phase demodulation techniques, also the algebraic sign of the strain change is retrieved. This is crucial for the intended applications in structural health monitoring and modal analysis. A linear strain response down to 47.5 pε and strain noise of 100 pε/√Hz is demonstrated for repetition rates in the kHz range. A field application of a vibrating bridge is presented. Our approach provides a cost-effective high-resolution method for structural vibration analysis and geophysical applications.

Numerical Investigation on Ultra-Broadband Tunable Microwave Frequency Comb Generation Using a Semiconductor Laser Under Regular Pulse Injection

A system scheme for generating ultra-broadband microwave frequency comb (MFC) based on a semiconductor laser under optical injection of regular pulse from a current-modulated laser is proposed, and the performances of the generated MFC are numerically investigated. In such a scheme, a distributed feedback semiconductor laser (DFB-SL1) under current modulation with suitable modulated parameters can be driven into regular pulsing state, which is utilized as a seed MFC with tunable frequency interval to inject into another distributed feedback semiconductor laser (DFB-SL2) for producing optimal MFC with ultra-broadband and tunable frequency interval. The simulated results demonstrate that for a seed MFC with a bandwidth of 26.4 GHz (27.0 GHz) and frequency interval of 3.3 GHz (2.7 GHz), the bandwidth of the optimized MFC generated by DFB-SL2 can arrive at 72.6 GHz (75.6 GHz) through selecting appropriate injection parameters.

Self-mixing interferometry for distance measurement using a semiconductor laser with current-modulated wavelength

The capabilities of the method of current-modulated laser wavelength are shown to be extended by using harmonic modulation. Advantages of the sawtooth modulation method associated with using the frequency measurements of the interference signal as an information parameter have been researched. Advantages of the harmonic-type modulation based on the use of measurement results for spectral component amplitudes of the interference signal for distance determination have also been investigated. A comparative analysis of methods for absolute distance measurements at the existing capabilities of semiconductor laser wavelength modulation has demonstrated the benefits of the harmonic modulation of laser diode current for distances under 30 cm, showing the sawtooth current modulation to work better for distances greater than 50 cm.

An interferometric system for measuring thickness of parallel glass plates without 2π ambiguity using phase analysis of quadrature Haidinger fringes.

An interferometric system is proposed for measuring the thickness of parallel glass plates by analyzing Haidinger fringes. Although a conventional Haidinger interferometer can measure thickness without 2π ambiguity using positions of peak and valley points in the interferogram, measurement accuracy is directly affected by the number of these points involved in the calculation. The proposed method obtains phase values over the entire interferogram by analyzing the quadrature Haidinger fringes generated by a current modulated laser diode. Therefore, it can achieve high speed measurement and nanometric resolution without mechanical rotation and thickness limitation of specimens. In the experiments, the standard deviation of repeated thickness measurement was evaluated as less than 0.3 nm, and the measured thickness profile of the proposed system agreed with that of a conventional thickness interferometer within ±15 nm. We also discussed the required accuracy of refractive index of specimens to implement the proposed method successfully and presented an exemplary measurement result of a multi-layer coated sample having a discontinuous thickness profile.

Measurement and analysis of a piezoelectric mirror shifter impulse response using self-correcting synthetic-heterodyne demodulation based Michelson interferometer vibrometer with gain and phase control feedback

Laser Doppler vibrometry is often employed for non-contact vibration measurements. A test beam scattered from a target is interfered with a reference beam on a photodiode and the resulting signal is processed to determine the target vibration. Most commercial vibrometers use a phase modulator in one branch of the interferometer. The requirement for a phase modulator can be removed by the use of synthetic-heterodyne demodulation. The optical source is a sinusoidal current modulated laser diode resulting in a frequency modulation of the input lightwave. The photocurrent has the form of the cosine of a carrier at the modulation frequency plus the dynamic phase difference between the beams, the time differential of which is proportional to the vibration. The detected signal spectrum comprises bands centered at integer multiples of the modulation frequency, which are processed to retrieve the vibration. We describe a new self-correcting synthetic-heterodyne technique employing phase and gain feedback, which is significantly less sensitive to the received optical power. The system, which has a frequency range of 0.2-9 kHz, is used to measure the impulse response of a piezoelectric mirror shifter. The vibration signal is analyzed using Hilbert transform techniques to determine the shifter resonant frequencies and decay rates.

Simultaneous measurement of strain and temperature using a Fabry–Perot interferometer consisting of Bragg gratings in polarization-maintaining fiber and current-modulated laser diodes

A fast and high-resolution simultaneous measurement of strain and temperature using an optical fiber sensor is presented. Temperature and strain can be measured simultaneously by using two types of reflection spectra of a Fabry–Perot interferometer consisting of fiber Bragg gratings in a polarization-maintaining fiber (PM-FBG-FPI). The fine structure of a reflection spectrum of the PM-FBG-FPI enables the high-resolution detection of wavelength shifts. We present a fast interrogation method with current modulation of a laser diode for PM-FBG-FPI sensors. The resulting fast measurement is demonstrated experimentally.

HAMR Thermal Gradient Measurements and Analysis

The edge gradient of the thermal spot used to define bits in heat-assisted magnetic recording is a critical parameter in determining the quality of the magnetic transitions. We review the laser current modulation method used to measure this parameter and introduce the sideband ratio (SBR) approach as a fast, inexpensive technique for getting data on large volumes of heads or media. Mathematical derivation of the SBR method is given along with the details of implementation. Production level data on thousands of recording heads are used to illustrate its usefulness.

Active mode-locking in semiconductor lasers

This paper presents the results of experimental studies of active mode-locking in an external-cavity semiconductor laser. Two methods of obtaining active mode-locking regimes have been studied: by modulation from an external generator with a frequency close to the intermode frequency of the external laser cavity and by laser current modulation at the frequency of intermode beats of the external laser cavity through an amplification feedback loop. In the first case, a stability of intermode beats of 10-14 is experimentally achieved, and in the second case, it is shown that semiconductor laser current modulation by an amplified intermode beat signal from the output of a photodetector provides a stable mode-locking regime also with mode frequency tuning, i.e., a mode-locking regime involving intermode frequency tuning with optical cavity tuning. The effects that accompany mode locking, in particular, the pulling effect and the stability of the mode-locking regime, and possibility of obtaining highly stable intermode beats by suppressing the amplitude noise component in the modulating signal have been studied. A phenomenological study of the pulling and phase locking of the intermode frequency is attempted.

Sub-megahertz frequency stabilization of a diode laser by digital laser current modulation

Digital laser current modulation (DLCM) is a convenient laser stabilization scheme whose major advantages are simplicity and inexpensiveness of implementation. However, there is a tradeoff between the SNR of the error signal and the laser linewidth due to the direct laser frequency modulation. In this paper, we demonstrated that DLCM can reduce the FWHM linewidth of a tunable diode laser down to 500 kHz using the modulation transfer spectrum of D2 line of a Li6 atomic vapor. For this purpose, a theoretical model is provided to analyze the DLCM-based modulation transfer spectrum. From the analysis, we experimentally explored the modulation effect on the DLCM spectrum to minimize the laser linewidth. Our result shows the optimized DLCM can stabilize a diode laser into the sub-megahertz regime without requiring acousto-optic and electro-optic modulators.

An improved sensitivity normalization technique of PGC demodulation with low minimum phase detection sensitivity using laser modulation to generate carrier signal

The phase carrier signal of the PGC demodulation in fiber optic interferometric sensors (FOIS) may be generated by using laser current modulation to achieve all-fiber structures in long-distance and outdoor applications. An optical intensity modulation with carrier frequency induced from the laser modulation will cause the instability and distortion in the PGC demodulation output. Additionally, since the optical intensity attenuation of the FOIS will degrade the PGC demodulation output, proper sensitivity compensation on the PGC demodulator is necessary for the field applications. In this paper, we propose an improved technique by using a balanced photoreceiver in the FOIS to attain significant improvements in the instability and distortion of the PGC demodulation output. Moreover, we choose a small phase scrambler signal amplitude of 1rad to achieve improved the minimum phase detection sensitivity (MPDS) as well as sensitivity normalization because the improvement of the MPDS is very important to the advanced FOIS. The experimental results show that the improved values of the best MPDS are 5.7dB (for DFB laser diode) and 4.3dB (for EP1550 laser diode), respectively, by using the improved technique. From the sensitivity normalization experiment results, the relative error is shown to be less than 0.5% when the optical intensity attentuation is no more than 15dB. It verifies the effectiveness of the sensitivity normalization technique proposed in our improved PGC demodulation system.

Injection-locked diode laser current modulation for Pound-Drever-Hall frequency stabilization using transfer cavities

A phase modulated RF current source is applied to an injection locked diode laser operating at $780\unit{nm}$. This produces tunable phase modulated sidebands of the laser suitable for stabilizing the length of an optical transfer cavity using the Pound-Drever-Hall technique. The Pound-Drever-Hall signal is anti-symmetric about the lock point, despite the presence of significant diode laser amplitude modulation. The stabilized optical transfer cavity is used to frequency stabilize a 776 nm external cavity diode laser. The stability and tunability of this transfer cavity locked laser is established by observation of the hyperfine components of the $^{87}$Rb $5P_{3/2}-5D_{5/2}$ transition in a vapor cell.

Sensitivity normalization technique of PGC demodulation with low harmonic distortion and high stability using laser modulation to generate carrier signal

The phase carrier signal of the PGC demodulator can be generated by using laser current modulation to achieve all-fiber structures for long-distance and outdoor applications. However, an optical intensity modulation with carrier frequency induced from the laser modulation may cause the instability and distortion in the PGC demodulation output. In addition, optical loss may reduce the PGC demodulation output so sensitive that sensitivity compensation on demodulation circuit is needed. In this paper, we propose a novel phase scramble technique by using low frequency phase signal with amplitude 2.405 rad to achieve the sensitivity normalization, and improve the instability and distortion of the PGC demodulation output simultaneously. Experiment results show that without applying scramble signal, the maximum value of total harmonic distortion is very close to the theoretic value and it is large. After applying scramble signal which matches the theorectical optimum condition J 0( ϕ L ) = 0, the instability and distortion of the PGC demodulation output are reduced obviously. According to the sensitivity normalization experiment results, the relative error is less than 0.6% when the optical intensity attentuation is no more than 15 dB. Therefore, the sensitivity normalization technique proposed in our study is effective.