Current-modulated Laser Diode Research Articles

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.

Fast interrogation using sinusoidally current modulated laser diodes for fiber Fabry–Perot interferometric sensor consisting of fiber Bragg gratings

A fast interrogation method using a sinusoidal modulated laser diode for a fiber Fabry–Perot interferometric sensor consisting of Bragg gratings (FBG–FPI) is proposed.. The FBG–FPI has sharp transmittance peaks in the reflection band of the FBGs. Wavelength sweep produced by current modulation of a laser diode can be used to detect the peak position. This enables high-resolution strain or temperature measurement. To precisely control the current, the current modulation is realized using a laser diode controller (LDC) with external modulation function. In the modulation by a sawtooth wave, the possible speed of wavelength sweeping is limited to 100 kHz or less due to the bandwidth limitation of an LDC and thermal effect in a laser diodeUsing a sinusoidal wave as a modulation waveform enables wavelength sweeping at speeds exceeding 100 kHz. The modulation characteristics of the laser wavelength is evaluated experimentally and the operating wavelength is monitored using an asymmetric Mach–Zehnder interferometer. The resolution of 0.2 fm/\(\sqrt{\mathrm{Hz}}\) and measurement time of 1 \(\upmu\)s were experimentally demonstrated in the present sensor.

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.

An evaluation of a modulated laser encoder

Abstract This paper introduces a new method of interpolation for sub-nanometer-resolution linear encoders. This method, called SPPE (scanning position probe encoder), uses high-order harmonics information obtained by a sinusoidal scanning pickup located on a periodic grating surface. The proposed encoder uses a current-modulated laser diode with diffractive grating optics. Since the electrical current changes the laser-diode wavelength, the interference light intensity is modulated as a sinusoidal scanning pickup on the scale grating. Phase-detection circuits can decode the position information in the pickup signal by using phase-locked loop techniques. The decoder achieves an interpolation rate of over 1/40,000 with interpolation errors of less than ±1 nm. A new interpolation-error measuring system was developed for the encoder. Finally, the evaluation results reveal that the presented encoder shows both high resolution and strong robustness.

Investigation of trapped rubidium atoms through frequency-modulation- induced coherent transient effects

We investigated coherent transient oscillations in the transmission intensity of a frequency-modulated probe-laser beam through a sample of trapped atoms. The transients occur when the frequency of the probe-laser beam crosses a resonance of the atoms in a time of the order of the lifetime of the excited state. A theoretical description used previously to explain transient effects observed in Mössbauer spectroscopy has been developed further to adapt for the specific frequency- and amplitude-modulation characteristics of current-modulated laser diodes. The transients were recorded at various experimental parameters, and the results of the experiments were compared with results obtained from slow frequency sweeps and independent measurements of the modulation characteristics of the laser. The results show that the theory accurately explains the coherent transient phenomena.

Optoelectronic feedback stabilization of current modulated laser diodes

Optoelectronic feedback is proposed as a suitable method for output stabilization of current modulated laser diodes. Numerical simulations show low jitter operation with significant on/off ratios even beyond the relaxation oscillation frequency. Low frequency components in the output power spectra are efficiently suppressed.

Multiplexed heterodyne fiber interferometer that uses laser-diode modulation

An interferometer has been developed that uses a bundle of fibers and a current-modulated laser diode. Light from the laser diode is distributed to three fibers whose exit ends are fixed together, and fiber lengths are chosen so that their differences will not be equal. From the phase of separated heterodyne signals produced between the reference and the sensor fibers we can detect the change in optical path length that is caused in the sensor fibers by external perturbations. As an application, temperature sensing at two points has been demonstrated.

Laser-diode microscope with fiber illumination

Two new fiber-illuminated laser diode microscopes which generate weak speckle images are proposed. In the first microscope, the output of a current-modulated laser diode is passed through a single multimode fiber whose exit face illuminates a conventional microscope with a continuously changing radiation pattern. This type of illumination generates time-accumulated, uncorrelated or partially correlated speckle image patterns that are averaged by a video detector to reduce speckle noise. In the second microscope, the exit face of a modified fiber bundle which functions as an extended secondary source consisting of N uncorrelated point sources illuminates the microscope. These point sources generate their own respective speckle image patterns that are ensemble-averaged by the detector to eliminate speckle noise. The modified fiber bundle is designed to ensure that the N point sources are mutually uncorrelated by making the length difference of any two fiber guides among the N fiber guides making up the bundle to be greater than the coherence length of the light source. Since the averaging of N speckle patterns in this arrangement is done in space domain (ensemble averaging), laser diode needs not be modulated. Both arrangements eliminate speckle noise without employing any moving mechanical parts, offer flexibility, compactness, and ease in adapting to conventional microscope.

Cascaded operation of arrays of symmetric self-electro-optic effect devices

Two identical optical modules were used to demonstrate the cascaded operation of 32-element arrays of symmetric self-electro-optic effect devices. The devices had 5 microm x 10 microm optical windows spaced on a square grid with a 20-microm spacing. They were operated as optically interconnected inverters at 1.1 MHz. The optical power was provided by two current modulated laser diodes per array, each with a maximum output power of 9 mW. The operation of the devices as logic gates is optically implemented but not demonstrated.

Output characteristics of RF-modulated laser diodes

The authors analyze theoretically and experimentally the output characteristics of periodically current-modulated laser diodes with an without an external resonator. It is shown that nonlinear output characteristics can be obtained without introducing an intensity-dependent gain saturation. The total emitted intensity per modulation period shows, as a function of the DC part of the pump current, a characteristic saturation behavior each time before the number of emitted pulses per modulation period increases. It is shown that a similar behavior for the light-current characteristics is obtained for the mode-locked laser in an external cavity and the solitary laser, even though the mechanisms which yield the multiple pulse emission in both cases are rather different. >