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Showing papers on "Laser Doppler vibrometer published in 1989"


Journal ArticleDOI
TL;DR: In this article, the same principle of operation, namely the detection of a Doppler shift, fD, in the light scattered from a vibrating target, is used for the measurement of normal-to-surface vibration of a solid surface.

117 citations


Patent
15 Feb 1989
TL;DR: In this paper, a method and apparatus for optically detecting transient motion from a scattering surface is presented, where a laser beam having a predetermined frequency is directed onto such a surface to scatter the laser beam and produce a scattered laser beam defining an optical wavefront and having an optical spectrum with a central peak at the laser frequency and a sideband on either side of the central peak.
Abstract: A method and apparatus for optically detecting transient motion from a scattering surface A laser beam having a predetermined frequency is directed onto such a surface to thereby scatter the laser beam and produce a scattered laser beam defining an optical wavefront and having an optical spectrum with a central peak at the laser frequency and a sideband on either side of the central peak The laser beam scattered by the surface is caused to interfere with a reference beam derived from the scattered laser beam and having an optical wavefront substantially matching the wavefront of the scattered beam and an optical spectrum with a single peak at the laser frequency and no sidebands, to obtain an optical signal which is detected and converted into an electrical signal representative of the transient motion The optical detection technique according to the invention has a broad frequency bandwidth and a large etendue providing greater sensitivity of detection, and is particularly useful for detecting small surface deformations of a material or workpiece subjected to ultrasonic energy

51 citations


Journal ArticleDOI
TL;DR: In this article, a self-mixing type laser Doppler velocimeter (SM-LDV) was used to determine the direction of target motion within a velocity range of 23 mm/s-23 m/s.
Abstract: A compact vector-velocimeter is described which uses a self-mixing type laser Doppler velocimeter (SM-LDV). It electrically identifies features of the sawtooth-like Doppler beat signal waveform to determine the direction of target motion within a velocity range of 23 mm/s-23 m/s. The range of velocity measured is 0.2 mm/s-34 m/s. The measured vector velocity of a reciprocally moving target as well as a rotating target agrees well with the theoretical value. >

48 citations


Proceedings ArticleDOI
J.H. Lau1, C.A. Keely1
TL;DR: In this paper, a laser Doppler vibrometer method is developed to measure the frequency of soldered and unsoldered leads of these four surface-mount components, and the corresponding mode shapes of the leads and solder joints are also provided for a better understanding of their dynamic characteristics.
Abstract: Vibration frequencies of soldered and unsoldered leads of surface-mount wide and narrow small outline integrated circuits (SOICs), plastic leaded chip carriers (PLCCs), and plastic quad flat packs (PQFPs) are calculated using the finite-element method. The corresponding mode shapes of the leads and solder joints are also provided for a better understanding of their dynamic characteristics. It is found that the fundamental frequencies of all the soldered leads are at least five times larger than those of the unsoldered leads. A laser Doppler vibrometer method is developed to measure the frequency of soldered and unsoldered leads of these four surface-mount components. Experiments and theory are in close agreement. The results presented should be useful for theoretically determining the reliability of solder joints under shock and vibration conditions, and for developing a solder joint inspection system for use in a controlled manufacturing process. >

39 citations


Patent
12 Jun 1989
TL;DR: In this paper, a laser power control system is described which detects the power of an emtted laser beam by splitting the beam using a beam splitter to form a detector laser beam which includes a limited portion of the beam.
Abstract: A laser power control system is disclosed which detects the power of an emtted laser beam. The detection is preferably accomplished by splitting the beam using a beam splitter to form a detector laser beam which includes a limited portion of the beam. The beam is preferably split after polarization to obvate changes in detected power associated with polarization changes which can occur in the laser. The detector beam can be detected either continuously or intermittently, such as by using a chopping wheel which intermittently passes and interrupts the detector beam. The detected laser beam power signal is compared against a predetermined set point or preprogrammed time variable laser power profile thus allowing desired laser power to be achieved. The laser beam is also preferably equalized to achieve a more uniform cross-sectional laser power density. Laser beam equalizers are shown which disperse a laser beam into non-parallel rays which are passed through an optical channel, such as a reflective passage or optical fiber. The system provides laser beams having controlled and equalized laser power.

39 citations


Patent
01 Mar 1989
TL;DR: In this article, the authors propose an approach for combining two laser beams each having a power level into a common colinear laser beam, which is defined as the sum of the power levels of the two different laser beams comprising a support, a polarization selective device disposed on the support having a first surfaces upon which a first of the laser beams impinges for transmitting the first laser beam with substantially full power transmission along an axis and having a second surface upon which the second laser beam impinges to reflect the second one's power along the axis.
Abstract: Apparatus for combining two laser beams each having a power level into a common colinear laser beam having a power level that is the sum of the power levels of the two laser beams comprising a support, a polarization selective device disposed on the support having a first surfaces upon which a first of the laser beams impinges for transmitting the first laser beam with substantially full power transmission along an axis and having a second surface upon which the second laser beam impinges for reflecting the second laser beam with substantially full power along the axis, resulting in a common colinear beam along the axis, and suitable optical and/or mechanical devices disposed on the support for providing the first and second laser beams to the polarization selective device, the first and second laser beams being substantially orthogonally polarized with respect to each other.

38 citations


Journal ArticleDOI
28 Mar 1989
TL;DR: In this article, the authors used finite element analysis to verify the acoustic emission (AE) measurements of the laser Doppler vibrometer and AE (acoustic emission) measurements and determined that the lowest natural frequency of a mini-Winchester slider is around 130 kHz.
Abstract: The natural frequencies of the various transducers and sliders were studied by finite element modeling and experimentation. Finite element analysis shows that the lowest ringing frequency of a 3370 slider is around 310 kHz. This value has been verified by the laser Doppler vibrometer and AE (acoustic emission) measurements. It is determined that the lowest natural frequency of a mini-Winchester slider is around 130 kHz. The 140-kHz frequencies reported in the literature using the two-rail sliders with piezoelectric transducers attached to the slider and the 160-kHz frequencies reported using AE transducers attached to the suspension mount are associated with the natural frequencies of these transducers and not of the sliders. >

29 citations


Journal ArticleDOI
TL;DR: In this article, the first observation of the deceleration of a Na-beam by a counterpropagating beam of a mode-locked laser was reported, and the results, the experimental setup and the single-mode laser system with its stabilisation were described.

29 citations


Journal ArticleDOI
TL;DR: In this article, the frequencies of AlGaAs laser diodes were stabilized to the F=4→F'=5 component of the Cs-D2 line with the Zeeman modulation method, in which the laser frequency remains unmodulated.
Abstract: The frequencies of AlGaAs laser diodes were stabilized to the F=4→F'=5 component of the Cs-D2 line with the Zeeman modulation method, in which the laser frequency remains unmodulated. Allan variance measurements were made on the beat note between two laser diodes thus stabilized and a frequency stability of 2.4×10-10 was obtained at an averaging time of 1s. By changing the dc magnetic field applied to the Cs cell, the frequency of the laser diodes could be tuned almost linearly in the range of -180 MHz to 180 MHz.

27 citations


Patent
08 Sep 1989
TL;DR: In this article, a collimated and focused laser measuring beam, intensity-modulated with a frequency of at least 200 kHz, was used to impinge on the workpieces at a distance of maximum 10 mm from the central longitudinal axis of the laser welding beam.
Abstract: A laser welding process includes the steps of deflecting, with an oscillating frequency of at least 100 Hz, a collimated and focused laser measuring beam, intensity-modulated with a frequency of at least 200 kHz, across the path of the weld groove during co-travel with the laser welding beam along the weld groove; directing the laser measuring beam to the workpieces; causing the laser measuring beam to impinge on the workpieces at a distance of maximum 10 mm from the central longitudinal axis of the laser welding beam; sensing, by an electro-optical sensor having a single light-sensitive element, rays of the laser measuring beam reflected by the workpieces; generating, by the sensor, measuring signals in response to the sensed reflected rays; applying follow-up signals in response to the measuring signals, and correcting the position of the laser welding beam by the follow-up device in response to the follow-up signals applied thereto.

27 citations


Patent
19 Sep 1989
TL;DR: In this paper, a laser beam scanning device including a plurality of laser beam sources for emitting laser beams having properties different from each other, respectively such that each of the laser beams is modulated by an image signal, a deflection member for simultaneously deflecting the laser beam emitted by the laser sources, a light detecting member for detecting one of the LBSs such that modulation of the LM beams emitted by LMBSs is started in response to a signal outputted from the light detecting members and an optical member which is disposed on an optical path from the deflection members to
Abstract: A laser beam scanning device including a plurality of laser beam sources for emitting laser beams having properties different from each other, respectively such that each of the laser beams is modulated by an image signal, a deflection member for simultaneously deflecting the laser beams emitted by the laser beam sources, a light detecting member for detecting one of the laser beams such that modulation of the laser beams emitted by the laser beam sources is started in response to a signal outputted from the light detecting member and an optical member which is disposed on an optical path from the deflection member to the light detecting member and optically selects only the one of the laser beams so as to irradiate the one of the laser beams onto the light detecting member.

Patent
26 Jan 1989
TL;DR: In this article, the laser beam energy applied to the liquid crystal can be maintained constant, thus enabling a constant written line width even during acceleration and deceleration of the optical axis control unit.
Abstract: The liquid crystal is written with a laser beam. The laser beam generated by a laser source is applied through an optical control unit such as galvanometer mirrors to the liquid crystal. The intensity of the laser beam applied to the liquid crystal is controlled by a laser beam intensity varying unit. A writing control unit controls the mechanical displacement caused by the optical axis control unit and controls the laser beam intensity varying unit. The laser beam energy applied to the liquid crystal can be maintained constant, thus enabling a constant written line width even during acceleration and deceleration of the optical axis control unit.

Journal ArticleDOI
TL;DR: To understand the fundamental limit of performance with a given laser radar system, the phase noise of a testbed laser radar has been investigated and a model for the signal variations from diffuse targets has been developed.
Abstract: To understand the fundamental limit of performance with a given laser radar system, the phase noise of a testbed laser radar has been investigated. Apart from the phase noise in the transmitter laser and the local oscillator laser, additional phase noise was introduced by vibrations caused by fans in power supplies and cooling systems. The stability of the mechanical structure of the platform was also found to be of great importance. Furthermore, a model for the signal variations from diffuse targets has been developed. This model takes into account the stray light, the speckle decorrelation, and Doppler shift due to moving targets.

Patent
01 May 1989
TL;DR: In this article, an apparatus for optically measuring the roughness of a surface of a subject in a non-contacting manner, based on a heterodyne interference, is described.
Abstract: An apparatus for optically measuring the roughness of a surface of a subject in a non-contacting manner, based on a heterodyne interference wherein a phase of a beat beam produced by a reference laser beam, and a measuring laser beam reflected by the surface of the subject and having a wavelength different from that of the reference laser beam is changed with a change in the length of an optical path of the measuring laser beam, which occurs due to a change in the surface roughness of the subject. The apparatus includes a laser source for producing the reference and measuring laser beams, a lens device having an object lens for converging the measuring laser beam on the subject surface, and a deflector for deflecting the measuring laser beam before the measuring laser beam is incident upon the subject surface, whereby the point of convergence of the measuring laser beam is moved on the subject surface in a direction parallel to the surface, while the subject is held stationary.

Journal ArticleDOI
TL;DR: In this article, a method for improving coherence in semiconductor laser by negative electrical feedback is proposed for stabilization of the center frequency of the field spectrum, linewidth reduction of the spectrum, frequency tracking to another highly coherent laser, and stable and wideband frequency sweep.
Abstract: A method for improving coherence in semiconductor lasers by negative electrical feedback is proposed for stabilization of the center frequency of the field spectrum, linewidth reduction of the field spectrum, frequency tracking to another highly coherent laser, and stable and wideband frequency sweep. Experimental center frequency stabilization of the master laser showed that the magnitude of frequency fluctuations was reduced to 50 kHz at the integration time tau =3 s. The linewidth of the master laser was reduced to 100 kHz, which was 1/50 that of the free running laser. Under these frequency control conditions, the frequency of the slave laser was controlled so that the phase of the heterodyne signal between the master and the slave lasers could be locked to that of a stable microwave synthesizer. The slave laser frequency tracked accurately to the master laser frequency. >

Patent
Erwin R. Wechsler1
18 Sep 1989
TL;DR: In this article, a circuit used in laser scanners which allows the continuous calibration of a laser so that the output power of the laser is accurately modulated by the input video signal regardless of temperature changes or aging effects of the LM.
Abstract: A circuit used in laser scanners which allows the continuous calibration of a laser so that the output power of the laser is accurately modulated by the input video signal regardless of temperature changes or aging effects of the laser. The video signal includes a test segment which has two steps. The light output of the laser is measured during both steps. The lower step measurement is compared to a reference and the error is used to adjust the bias current of the laser. The difference between the upper step measurement and the lower step measurement is compared to a second reference and the error is used to adjust the gain of the video signal so that the variation in laser power maximum and minimum remains calibrated.

Patent
25 Oct 1989
TL;DR: An output stabilizing apparatus for an optical head having a semiconductor laser, a beam splitter, an optical projection system, a monitor photodiode and an automatic laser power control circuit is described in this paper.
Abstract: An output stabilizing apparatus for an optical head having a semiconductor laser, a beam splitter, an optical projection system, a monitor photodiode and an automatic laser power control circuit. A main laser beam emitted from the semiconductor laser is split into two beams by the beam splitter. The monitor photodiode is disposed to receive one of the beams and generates a monitor signal. In response to the monitor signal, the automatic laser power control circuit controls the semiconductor laser intensity.

Patent
14 Dec 1989
TL;DR: In this paper, the second derivative of the optical light output of a laser with respect to the current through the laser, d²L/dI², is used in a feedback loop to control the laser bias current.
Abstract: A laser biasing stabilization circuit and method uses the second derivative of the optical light output of a laser with respect to the current through the laser, d²L/dI², in a feedback loop to control the laser bias current through the laser. The modulation current in the laser is controlled using a second, average power feedback loop.

Patent
Masayuki Tomoyasu1
27 Feb 1989
TL;DR: In this paper, an acousto-optic modulator provided on an optical path for intensity-modulating a laser beam in accordance with an input signal was used as a laser probing apparatus.
Abstract: A laser beam scanning system is used as a laser probing apparatus. The apparatus includes an acousto-optic modulator provided on an optical path for intensity-modulating a laser beam in accordance with an input signal, a beam intensity control device for measuring a beam intensity distribution of the laser beam incident on a wafer surface, for correcting the input signal to the modulator as a result of measurement, and for making uniform the laser beam illumination distribution over the wafer surface, and a device for measuring an electrical variation in that wafer pattern portion illuminated with the laser beam.

Patent
21 Dec 1989
TL;DR: In this paper, a single laser beam scanning system consisting of a collimator lens unit, a PLZT element for rotating the polarized direction of the laser beam over 90 degrees, a polygonal mirror causing the laser beam to sweep in a lateral direction, a scanning lens unit for regulating the liner velocity of sweeping motion, a polarization beam splitter transmitting or deflecting the beam toward a first destination depending upon the polarized directions, and a reflective mirror directing the laser beacon transmitted from the beam splitters to a second destination, is applied with a biasing voltage at
Abstract: For tracing two scanning paths with a single laser beam, a laser beam scanning system comprises a laser beam source radiating a laser beam, a collimator lens unit, a PLZT element for rotating the polarized direction of the laser beam over 90 degrees, a polygonal mirror causing the laser beam to sweep in a lateral direction, a scanning lens unit for regulating the liner velocity of sweeping motion, a polarization beam splitter transmitting or deflecting the laser beam toward a first destination depending upon the polarized direction and a reflective mirror directing the laser beam transmitted from the beam splitter to a second destination, and the PLZT element is applied with a biasing voltage at every second scanning motion for the rotation of the polarized direction, so that the laser beam rapidly changes the scanning path.

Patent
21 Jun 1989
TL;DR: In this paper, a laser transmitting and receiving system includes doppler compensation for large Doppler shifts in frequency caused by relative motion between a collocated transceiver and a target or relative motion among separated transmitters and receivers, which is provided by a feedback loop which may include rf components, a timing trigger from the computer, or wavemeters.
Abstract: A laser transmitting and receiving system includes doppler compensation for large doppler shifts in frequency caused by relative motion between a collocated transceiver and a target or relative motion between separated transmitters and receivers The system includes a tunable laser, a dithered laser optical frequency standard, and a computer for calculating the estimated doppler shift at a given time using platform navigation and attitude control inputs as well as inputs relating to pointing data controlling the direction of transmission The frequency standard and the computer output are employed to develop a doppler compensation signal which may be used to shift the frequency of the laser transmitter or shift the bandpass of a laser receiver filter The doppler compensation is provided by a feedback loop which may include rf components, a timing trigger from the computer, or wavemeters

Patent
20 Nov 1989
TL;DR: In this article, a method for measuring the energy of individual laser pulses or a series of laser pulses by reading the output of a PZ transducer which has received a known fraction of the total laser pulse beam was presented.
Abstract: A method for measuring the energy of individual laser pulses or a series of laser pulses by reading the output of a piezoelectric (PZ) transducer which has received a known fraction of the total laser pulse beam. An apparatus is disclosed that reduces the incident energy on the PZ transducer by means of a beam splitter placed in the beam of the laser pulses.

Journal ArticleDOI
TL;DR: In this article, the benefits obtained by using pulsed laser diodes with repetition rates much higher than the Doppler frequency are described and the advantages over cw and long-pulse operation are shown.
Abstract: The benefits obtained by using pulsed laser diodes with repetition rates much higher than the Doppler frequency are described and the advantages over cw‐ and long‐pulse operation are shown. These benefits are achieved by several characteristics of pulsed laser operation. First, in pulsed mode the time‐averaged number of photons may be considerably higher than in cw‐mode. Second, for some of the LDA data acquisition systems the laser pulses can be synchronized with the sampling process of the acquisition systems, and third, in multiple‐component systems the phases of the laser pulses in the emitter parts can be shifted with respect to each other so that one single receiver part is sufficient for all components and an identical wavelength can be used for all components. Experimental verification proves the feasibility of these conceptions.

Patent
22 Mar 1989
TL;DR: In this paper, a laser Doppler anemometer with a wavelength-stabilized laser diode whose beam after splitting is so concentrated in a definite area of a particle-containing flow of a fluid with the velocity that in this area an interference fringe pattern is produced, and with at least one semiconductor photodiode on which the laser beams (10, 11) progressing from the area (9) are refocussed after elimination of scattered light fractions by a scattered light trap (diaphragm stop) (14), for the detection of the difference signal I
Abstract: Laser Doppler anemometer with a wavelength-stabilized laser diode (1) whose beam after splitting (7) is so concentrated in a definite area (9) of a particle-containing flow of a fluid with the velocity (17) that in this area (9) an interference fringe pattern is produced, and with at least one semiconductor photodiode (13) on which the laser beams (10, 11) progressing from the area (9) are refocussed after elimination of scattered light fractions by a scattered light trap (diaphragm stop) (14), for the detection of the difference signal I between incident light intensity I o and scattered light intensity I scatter which the particles contained in the fluid emit into the whole solid angle 4π, where I=I o -I scatter By detection of the laser beam of the intensity I which is modulated with the Doppler frequency after having passed through the point of measurement (9), the whole light intensity I scatter emitted by the particle into the solid angle 4π can be utilized for the signal-to-noise ratio at the photodetector, and if suitable semiconductor photodiodes are used, the direction of flow can be identified

Patent
20 Jun 1989
TL;DR: In this article, a photodetector is used to generate an electrical signal which is proportional to the power level of the laser beam, and the electrical signal is converted to a high frequency modulated signal which controls a single acousto-optical modulator.
Abstract: A laser power beam control system includes a photodetector for generating an electrical signal which is proportional to the power level of the laser beam. The electrical signal is converted to a high frequency modulated signal which controls a single acousto-optical modulator. The acousto-optical modulator is in the laser beam path and controls the power level of the beam in response to the amplitude modulated signal. The system is particularly adapted to control a laser beam used in raster or vector scanning a laser for the formation of semiconductor integrated circuits.

Patent
03 Nov 1989
TL;DR: A self-focusing optical head of the type used in magneto-optical and optical recording systems for directing a focused laser beam onto a moving recording surface uses a non-achromatized optical path between the laser and recording surface and the laser wavelength (mode) shifts induced by optically coupling reflected laser energy back to the laser cavity to automatically maintain the beam in sharp focus (i.e., self-focus) on the recording surface as discussed by the authors.
Abstract: A self-focusing optical head of the type used in magneto-optical and optical recording systems for directing a focused laser beam onto a moving recording surface uses a non-achromatized optical path between the laser and recording surface and the laser wavelength (mode) shifts induced by optically coupling reflected laser energy back to the laser cavity to automatically maintain the beam in sharp focus (i.e., self-focus) on the recording surface. A focus servo system including a photodetector positioned in the path of reflected laser energy serves to coarsely position a focusing lens relative to the recording surface. A field lens, used to concentrate reflected laser energy onto the photodetector, is achromatized to compensate for the chromatic aberration between laser and photodetector. Such achromatization allows the focus servo to operate independently of the wavelength (mode) shifts required to achieve self-focusing of the laser beam on the recording surface.

Journal ArticleDOI
S. Yoneoka1, T. Owe1, Keiji Aruga1, Tomoyoshi Yamada1, M. Takahashi1 
28 Mar 1989
TL;DR: In this article, a measuring technique that can totally analyze the flight stability and access ability of the slider was developed, which involves three-dimensional modal analysis of the suspension using a laser Doppler anemometer, a laser doppler vibrometer and simultaneous measurement of spacing fluctuation.
Abstract: Heads for high-performance disk storage must be able to withstand an acceleration of dozens of G-force and be capable of positioning to submicron precision. A measuring technique that can totally analyze the flight stability and access ability of the slider was developed. It involves three-dimensional modal analysis of the suspension using a laser Doppler anemometer, a laser Doppler vibrometer, and simultaneous measurement of spacing fluctuation. Using this system to examine an inline head, it was found that notable lateral and torsional modes of the suspension vibration were caused by the access acceleration of the rectangular wave, and that there is pronounced spacing fluctuation. In addition, these lateral vibrations cause the pivot to slip, which may cause problems in high-speed head access. >

Patent
25 Sep 1989
TL;DR: In this article, a surface profile measuring device utilizing optical heterodyne interference comprising a laser source for emitting a laser beam, an object lens for converging the laser beam on the surface of an object to be measured, the laser being reflected from the surface surface of the object while provided with information on surface profile of the objects, an optical heterodermi-nous interference device for allowing the reflected beam to interfere with a reference beam having a frequency different from that of the reflected beacon to generate beat signals, a circuit for measuring the surface profile on the basis of phase variation
Abstract: The surface profile measuring device utilizing optical heterodyne interference comprising a laser source for emitting a laser beam, an object lens for converging the laser beam on the surface of an object to be measured, the laser beam being reflected from the surface of the object while provided with information on a surface profile of the object, an optical heterodyne interference device for allowing the reflected beam to interfere with a reference beam having a frequency different from that of the reflected beam to generate beat signals having the information on the surface profile of the object, a circuit for measuring the surface profile of the object on the basis of phase variation of the beat signals, a driving device including a piezoelectric element for relatively moving one of the object lens and the object to be measured in the direction of the optical axis of the laser beam incident on the surface, and a control device for controlling the driving device on the basis of the phase difference or amplitude ratio between the beat signals so that the surface of the object is disposed substantially at a focusing position of the object lens.

Journal ArticleDOI
TL;DR: In this paper, a theoretical analysis of a Doppler beat waveform was carried out, and its approximate solution was obtained by using the perturbation method, and the theoretical characteristics based on this solution agreed fairly well with the experimental ones.
Abstract: To explain the experimental characteristics of a laser Doppler velocimeter using the self-mixing effect of a semiconductor laser diode (LDV) with a rotating disc as a target, a theoretical analysis of a Doppler beat waveform was carried out, and its approximate solution was obtained by using the perturbation method. The theoretical characteristics based on this solution agreed fairly well with the experimental ones. The operating conditions of the LDV required for forming the beat wave into a sawtooth waveform, and the reasons why the sawtooth wave is transformed into a mirror image depending on the sign of the Doppler frequency were found, i.e., the phase angles of the second and third higher harmonics of the beat wave are inverted by the inversion of the direction of rotation. The upper limit of the Doppler frequency and the range of R parameter of the LD were estimated theoretically.

Patent
14 Jun 1989
TL;DR: In this paper, an excitation system for exciting a semiconductor laser device used in a laser printer is described, where a bias signal is superimposed on the modulated input signal and the resultant signal is applied to the semiconductor device for exciting the same.
Abstract: An excitation system for exciting a semiconductor laser device used in a laser printer. An input terminal receives a modulated input signal having a level changed depending upon information. A bias signal is superimposed on the modulated input signal and the resultant signal is applied to the semiconductor laser device for exciting the same. A photodiode is provided to receive the laser beam emitted from the semiconductor laser device to thus detect the intensity of the laser beam. In response to the detected intensity of the laser beam, the level of the bias signal is adjusted so that a predetermined quantity of laser beam is emitted from the semiconductor laser device when the modulated input signal having a possible maximum level is inputted to the input terminal.