Phase-locked phase-shifting laser diode interferometer with photothermal modulation

We propose a phase-shifting interferometer that uses both phase-locked and photothermal modulating techniques. In this interferometer the measurement accuracy is not affected by the intensity modulation that usually appears in current modulation. The surface profile of a diamond-turned aluminum disk was measured; the rms repeatability obtained was lambda/460.     

Two-wavelength laser diode interferometer with time-sharing sinusoidal phase modulation

We describe an interferometer system that uses two separate wavelengths to measure step height. The overlapping interference images detected by a CCD camera are easily separated by an ordinary integrating-bucket method and time-sharing sinusoidal phase modulation, in which two laser diodes are alternately modulated with a sinusoidal signal. A phase map is obtained only for the laser diode into which the modulation signal is injected. In this instance, a 1-microm step height was accurately detected.     

Superluminescent diode interferometer using sinusoidal phase modulation for step-profile measurement

We propose an interferometer in which the relationship between the degree of coherence (DCH) and the optical path difference (OPD) is utilized for determining an OPD longer than a wavelength. A superluminescent diode is employed as the source of the interferometer, and sinusoidal phase-modulating interferometry is used to detect the DCH and the phase of the interference signal. The combination of the OPD determined from the DCH and the phase of an interference signal enables us to measure an OPD longer than a wavelength with a high accuracy of a few nanometers. Experimental results show clearly the usefulness of the interferometer for a step-profile measurement.     

Wavelength-multiplexed phase-locked laser diode interferometer using a phase-shifting technique

We propose a new signal-processing method for eliminating measurement errors that occur in the wavelength-multiplexed phase-locked laser diode interferometer. The basic idea proposed here is a very simple but effective way to improve measurement accuracy. With our scheme, the phase in the interference signal is strictly shifted by 2pi, which enables us to eliminate measurement errors. The equivalent wavelength ? is 80 mm, and the measurement accuracy reaches ~?/600. A step-height measurement was also carried out in the experiment.     

Distance- and velocity-detection interferometer by using a frequency triangular-modulated laser diode

A scheme for distance and velocity signal detection is implemented. This technique is based on a laser diode with its frequency modulated by a triangular waveform and followed by time gating.     

Sinusoidal phase-modulating interferometer insensitive to intensity modulation of a laser diode for displacement measurement

In sinusoidal phase-modulating laser diode interferometers, an injection current of a laser diode is sinusoidally modulated to scan the laser wavelength. However, the modulation of the injection current also involves an intensity modulation of the light, which increases the measurement error if a conventional signal processing is used. A novel signal processing for displacement measurement is proposed to eliminate the influence of the intensity modulation. Numerical simulation results and experimental results make it clear that an optimal depth of the sinusoidal phase modulation exists that can reduce the measurement error to a few nanometers.     

Phase-shifting birefringent scatterplate interferometer

We realized what we believe is a new phase-shifting scatterplate interferometer by exploiting the polarization characteristics of a birefringent scatterplate. The common-path design of the interferometer reduces its sensitivity to environmental effects, and phase shifting allows quick and accurate quantitative measurements of the test surface. A major feature of the birefringent scatterplate approach for phase shifting is that no high-quality optical components are required in the test setup. The theory of the interferometer is presented, the procedure for the fabrication of the birefringent scatterplate is described, and experimental results are shown.     

Phase-shift-locked interferometer with a wavelength-modulated laser diode

A phase-shift-locked interferometer has been constructed for distance measurement. A phase shift produced by sawtooth-current modulation of a laser diode is locked to a phase difference preset by polarization optics that consists of a quarter-wave plate and polarizers through an electrical feedback technique. An optical path difference can be measured from the locked sawtooth-wave current amplitude in real time. The sensitivity of the interferometer is discussed.     

Disturbance-free high-speed sinusoidal phase-modulating laser diode interferometer

A surface profiler that incorporates a feedback controller to eliminate external disturbances is proposed and demonstrated. Its overall performance is dependent on the frequency response of the feedback loop. The frequency of the modulating signal strongly influences the response of the feedback controller. When we used the integrating-bucket method, the CCD camera had to be operated at a low-frequency video rate. Our technique uses a CCD camera equipped with an electronic shutter. The shutter function enables us to apply high-speed sinusoidal phase modulation to the conventional integrating-bucket method under the standard video rate.     

Hyperfine structure measurement of rubidium atom and tunable diode laser stabilization by using Sagnac interferometer

The Rubidium saturated absorption spectra for D2 transition lines are used to measure the Fabry-Perot interferometer free spectral range (FSR). The scale linearity of the laser frequency tuning is determined. The Sagnac interferometer has been used for the laser stabilization. The result shows that the laser frequency is stabilized upto sub-mega Herz level. Also the hyperfine structure [5(2)S(1/2) F = 3 --> F' = 2, 3, 4 5(2)P(3/2) 85Rb] of the rubidium atom has been measured by using the tilt locking method, which shows the same result as the conventional saturation spectroscopy.     

一种时域解调激光干涉仪调制信号的方法

针对某低频水平振动校准设备的激光干涉仪,利用NI公司的多功能采集卡,采集激光干涉仪的调制信号及与其同相位的电容式位移传感器的输出信号.根据激光干涉仪调制原理,提出一种时域解调调制信号的方法.根据调制信号条纹的时域特性和它被限幅的实际特点,利用labview软件实现了信号的解调,并将电容位移传感器信号作为参考信号,还原解调后信号的相位,从而得到振动台高精度的振动波形.实验证明,此方法对连续振动信号的解调效果良好,具有较好的重复性和稳定性,精度可达到激光干涉仪发射激光波长的四分之一.     

Multipass Michelson interferometer with the use of a wavelength-modulated laser diode

An improved multipass Michelson interferometer is implemented. This technique uses the fact that the wavelength of a laser diode varies in proportion to the diode's injection current. With this method the sensitivity augmentation is accomplished by inserting a beam splitter into one arm of the interferometer, resulting in multiple reflections between the end mirror and the beam splitter. In addition, the interference of laser beams reflected from two arms can be accomplished with unequal arms in the condition of a short coherence length. The sensitivity increase of interference fringes and the compensation of the short coherence length have been demonstrated in experiments.     

Real-time displacement measurement with a two-wavelength sinusoidal phase-modulating laser diode interferometer

A two-wavelength interferometer that uses two separate modulating currents with different phases but the same frequencies to detect a greater degree of object displacement in real time is proposed and demonstrated. The measurement error was 57 nm, roughly 1/80 of the synthetic wavelength. We have confirmed that this modulating technique enables us to equip our prototype interferometer with a simple feedback-control system that eliminates external disturbance.     

Phase-shifting (Sagnac) interferometer with external phase control

A novel ring configuration for phase-shifting interferometry with external phase-shifting control is presented. The device is a polarization (ring) interferometer, in which the reference and test arms are parts of the same collimated beam. The key point is to manage the polarization of the light such that orthogonal linear polarizations describe counterpropagating paths in the ring interferometer. The phase shift between the two waves is externally controlled with a Pockels cell, which permits fast phase modulation without the need for moving parts inside the interferometer.     

Theory of digital modulation of semiconductor laser diode remotely

The authors present a new theory that can be applied to modulate the semiconductor laser diode (LD) remotely. The proposed scheme utilizes the effect of incoherent external optical feedback (EOF) on the LD output optical signal. In particular, owing to the high increase in threshold carrier density, the LD will be impelled to turn off when the value of external reflectivity is equal to the laser back reflectivity. Thus, by exposing the LD, which is injected with a dc-current higher than threshold, to incoherent EOF digital signal (which conveys the transmitted information), the output optical signal of LD can be modulated accordingly.     

Signals of an intermodal fiber interferometer induced by laser frequency modulation

Signals of an intermodal fiber interferometer induced by laser optical frequency modulation are studied. Dependences of signal amplitudes and spectra on the laser frequency deviation are examined theoretically and experimentally for various optical fibers. It is established that the sensitivity of the intermodal fiber interferometer to the laser frequency variation essentially depends on the fiber refractive index profile. The minimal sensitivity corresponds to graded-index (α ≈ 2) multimode optical fibers. Step-index optical fibers (α = ∞) are more sensitive to the laser frequency variation by more than a factor of 100.     

Phase-shifting interferometry with equal phase steps by use of a frequency-tunable diode laser and a Fabry-Perot cavity

A phase-shifting interferometry (PSI) with equal phase steps by use of a frequency-tunable diode laser and a Fabry-Perot cavity is proposed for the Carré algorithm. The measurement accuracy of the Carré algorithm depends on the equality of the phase steps. Using the Fabry-Perot cavity as a highly stable optical frequency reference, a high degree of phase step equality can be realized in PSI with an optical frequency shift. Our experimental scheme realizes an optical frequency step equality higher than 5.1 x 10(-5) and a measurement repeatability of lambda/800.