Abstract: A laser module capable of easy optical axis adjustment between the very small waveguide in a short period of time, and a method for adjusting an optical axis thereof. The laser module includes a laser device LD; an optical waveguide device SHG equipped with a waveguide; lenses L1 and L2 for converging light from the laser device LD and leading it into the waveguide o; and an actuator for positioning the focusing spot formed by the lenses L1 and L2 in a 3D direction. If the intensity of the emerging light coming from the waveguide is less than or equal to a predetermined reference value, the focusing spot is defocused. Then, the position of the focusing spot is adjusted in a plane perpendicular to the optical axis. Then, the focusing spot is adjusted to be in focus.

Abstract: A color dazzle system to enhance area denial to personnel mission for law enforcement, homeland security, border patrol, and the military effectively by interrupting the ability of combatants to continue with their intended mission. The color dazzle system has a multi-wavelength laser module for generate desired color or wavelength ranges of light, a beam projector module for pointing the light towards the target to be dazzled, and a fiber optic beam delivery system for delivering the light to the beam projector module. By adjusting the power output according to the corresponding wavelength and the distance to the target, and quickly switching between the wavelengths of light, a dazzle effect can be obtained with an irradiance within a safety range for the eye.

Abstract: A light source device includes plural light-emitting elements that emit lights, a wavelength selection element that has plural light selection areas in which wavelength selection is performed for the lights emitted from the plural light-emitting elements, respectively, and selectively reflects a part of the lights emitted from the plural light-emitting elements, a state detecting unit that detects states of the plural light selection areas of the wavelength selection element, and a state changing unit that changes, according to the states of the plural light selection areas detected by the state detecting unit, the states of the light selection areas of the wavelength selection element to make wavelengths of the lights selected in the plural light selection areas different from one another.

Abstract: A laser system (100) produces at least one laser beam (204) that is output from the laser system. The laser system includes a safety device (124) that terminates the output of the laser beam from the laser system. The termination occurs upon the laser beam interacting with a predetermined reflective surface (202) to change a characteristic thereof, thus indicating of a fault condition where the laser beam may cause damage to a human eye, for example. The safety device (124) may be reflective in the path of the laser beam(s) from source to output, and the reflective surface (202) melts or ablates upon the fault condition, thus interrupting the path and preventing laser output. In addition, the safety device (124), upon the fault condition, may produce or trigger a control signal (126) to turn off the laser sources (110). A sensor (620) may be located behind the predetermined surface (202) to indicate the fault condition upon ablation and/or melting of the predetermined surface (202).

Abstract: A wavelength locker for use at more than one wavelength includes filters with different characteristics for a corresponding detector. The filters may be etalons having different free spectral ranges, e.g., having different apparent or real thicknesses. If more than three such filters are used outputting offset periodic signals, a reference detector may be eliminated and continuous operation over a wavelength range may be realized.

Abstract: A femtosecond laser with stabilized output, and a method for stabilizing the output of a femtosecond laser. The femtosecond laser comprises a regenerative amplifier, a photodetector, a control portion and a variable attenuator, wherein the variable attenuator is provided so as to be capable of controlling an output beam of the regenerative amplifier, the photodetector is provided between the regenerative amplifier and the variable attenuator, the photodetector detects an output beam of the regenerative amplifier and transmits the results of the detection to a power controller, and the power controller controls the variable attenuator depending on the a difference between a predetermined target value and the results.

Abstract: A transmitting and receiving device, in which the received signal which is produced by the receiving device has only a small amount of crosstalk. This object is achieved by providing a transmitting and receiving device having a transmitting device for producing a transmission signal, a receiving device for producing a received signal, and a compensation device which is connected to the transmitting device and to the receiving device and which at least reduces any crosstalk which is produced by the transmitting device in the receiving device.

Abstract: Light-emitting system comprising an integrated control photodetector and method for fabricating said system. This system is particularly suitable for optical connections and comprises a light-emitting electronic component (2) and a light guide (4) that receives the light emitted by the component, said guide comprising a light input face (8) arranged facing the component and reflecting a part of the light it receives, and a photodetector (14) integrated with the component and that detects a part of the light emitted by said component, wherein said photodetector is placed near to the component and is capable of receiving a part of the light reflected by the light input face. The active layer of the light-emitting electronic component is made of the same material as the active layer of the photodetector.

Abstract: A method embodiment of the invention includes a scheme for trimming and compensation of a laser emitter in a fiber optic link. A laser emitter is provided. Also a reference optical power value is determined using data models of laser emitter performance generated by statistical analysis of sample population of lasers that are similar to the subject laser emitter. The performance of the subject laser is measured and qualified optical power levels for the subject laser emitter are determined using the reference operating power value and measured performance of the subject laser emitter. The driving current of the subject laser emitter is adjusted to compensate for changes in temperature until the optical power of the subject laser emitter obtains a desired qualified optical power level. Related methods of determining qualified optical power levels for a laser emitter in a fiber optic link are also disclosed.

Abstract: The present invention is to provide an optical transmitter that installs a laser diode and a circuit to control currents supplied to the laser diode, and reduces the size of the control circuit. The transmitter includes an LD, an LD-Driver, a monitor PD, and a control unit that outputs two data for the bias and modulation currents to the driver based on the photocurrent. The control unit provides a memory whose addresses correspond to the bias current, while a data for the modulation current is set within an address of the memory. Accordingly, the control unit adjusts the bias current so as to maintain the photocurrent constant and outputs the data for the modulation current to the driver by reading information stored in the address corresponding to the bias current.

Abstract: A device and method for monitoring a light beam (e.g., laser beam) without intruding upon the light beam is disclosed. The device comprises a focusing lens which receives scattered light from a reflective or transmitting optical element and directs the scattered light to a photo detector. The photo detector generates one or more electrical signals which are proportional to an intensity distribution of the light beam for analysis by a processor. The analyzed electrical signals are then used to adjust a light beam source accordingly. The device may further comprise a display and printer for visually depicting the light beam characteristics.

Abstract: A system and method for controlling an optical transmitter. Proximate to startup of the optical transmitter, the laser bias to the optical transmitter is increased from a minimum to a setpoint according to a predefined function. After the laser bias setpoint is reached, the electrical modulation amplitude is increased from a minimum to a setpoint according to a predefined function. Proximate to shutdown of the optical transmitter, the electrical modulation amplitude is decreased from the setpoint to the minimum according to a predefined function. After the electrical modulation amplitude minimum is reached, the laser bias to the optical transmitter is decreased from a setpoint to a minimum according to a predefined function.

Abstract: The present invention relates to opto-isolators. Opto-isolators are disclosed that include a transmitter package and a vertical VCSEL disposed within the transmitter package. The opto-isolators further include a receiver package and a photodetector disposed within the receiver package. The photodetector is optically coupled to the VCSEL and configured to receive an output optical signal generated by the VCSEL. The opto-isolators further include an alignment package configured to receive the transmitter package and the receiver package. In another embodiment, opto-isolators include a VCSEL and a photodetector optically coupled to the VCSEL and configured to receive an output optical signal generated by the VCSEL. The opto-isolators further include a package enclosing both the VCSEL and the photodetector.

Abstract: A method for determining a condition of the laser system includes determining a change in a laser current from an initial value. A method for measuring a laser current includes determining a difference between the values of a power supply current, which is the value of the laser current. A method for measuring a transmitted power includes generating a first control signal that sets a magnitude of a bias current supplied to a laser, generating a second control signal that sets a of a modulation current supplied to the laser, and determining a difference between values of a high and a low transmitted powers. A method for measuring a received optical power includes determining a received OMA corresponding to the power signal, which is the transmitted OMA minus a known loss through a calibration fiber that couples the laser transmitter to the laser receiver.

Abstract: PROBLEM TO BE SOLVED: to realize a wide bandwidth light source capable of a stable operation providing a less spectrum variation. SOLUTION: Optical pulses 202 outputted by this optical pulse-generating device 201 are split by optical coupler 210. The one of split optical pulses is inputted in white light-emitting device 204 to emit the white light having the wide bandwidth. The other split is inputted in optical band pass filter 212 in operation-stabilizing circuit 207. Optical detector 213 detects a signal level of a specific wavelength and input in feedback circuit 215 to generate first and second feedback signals 205 and 206. Ring resonator fiber laser 208 uses any one of them and controls a rotation angle of the waveplate to realize stabilization of the output.

Abstract: Methods and system are provided for automatic power control of a laser diode, e.g., in a laser driver. In accordance with an embodiment of the present invention, a power controller includes a detector circuit adapted to detect the output of the laser diode and to produce a measured output therefrom. A comparator compares a desired output to the measured output, and produces an error signal therefrom. The error signal is provided to an integrator circuit that produces an integrated error signal. At least one digital-to-analog converter (DAC) uses the integrated error signal to produce a current drive signal that drives the laser diode.

Abstract: The present photonic RF generation and distribution system provides a system and method for distributing an RF output signal. The photonic RF distribution system includes two optical sources for generating optical signals. A first optical source (42) is operable to generate a first optical signal having an operating frequency. A second optical source (44) is operable to generate a second optical signal having an operating frequency. A modulator (46) is operable to impress an RF modulation signal on a tapped portion of the first optical signal such that a modulated signal is generated. A first coupler (52) combines the modulated signal with a tapped portion of the second optical signal, thereby forming a combined signal having a difference frequency component. A control photodetector (50) is responsive to the combined signal to generate a tone signal.

Abstract: All optical clock recovery includes a transmitter for generating an optical timing signal. The transmitter includes a semiconductor laser for the production of a dynamically synchronizable timing signal, the laser having an external resonator for feedback of the timing signal to the laser, the feedback having a delay time greater than a relaxation oscillation time for the laser, and the laser outputting an optical timing signal having a characteristic dynamic. The transmitter supplies the optical timing signal to a receiver configured to receive the timing signal and to synchronize to the laser on receipt of the timing signal, such that the receiver outputs a recovered timing signal having the characteristic dynamic.

Abstract: It is an object to provide a laser apparatus, a laser irradiating method and a manufacturing method of a semiconductor device that make laser energy more stable. To attain the object, a part of laser beam emitted from an oscillator is sampled to generate an electric signal that contains as data energy fluctuation of a laser beam. The electric signal is subjected to signal processing to calculate the frequency, amplitude, and phase of the energy fluctuation of the laser beam.

Abstract: A surface emitting semiconductor laser includes a laminate of semiconductor layers emitting multimode laser light, and a block member blocking light of a specific mode among the multimode laser light emitted from the laminate.

Abstract: A laser diode driver, and driving method, for driving a laser diode in an optical recording/reproducing apparatus, having an auto laser power control (APC) operation, an optical pickup device, and an optical recording/reproducing apparatus, and method therefore, using the laser diode driver. The laser diode driver includes a multiplexer, a pulse generator, a differential amplifier, and an adder. The multiplexer selectively outputs drive potentials, such as a peak power drive potential, a bias power drive potential, a read power drive potential, and the like, corresponding to power levels of a laser signal, in response to a select signal applied to the multiflexer.

Abstract: The present invention is directed to a novel arrangement of optical devices for the rapid patterning of laser profiles used for desorption and/or ionization sources in analytical mass spectrometry. Specifically, the new optical arrangement provides for a user-defined laser pattern at the sample target that can be quickly changed (on a microsecond timescale) to different dimensions (or shapes) for subsequent laser firings.

Abstract: A temperature compensation system comprises a laser driver having a first potentiometer, the laser driver configured to provide a first signal to a laser based on a value of the first potentiometer and an optical communication analyzer configured to provide a second signal indicative of a first output parameter of the laser. The temperature compensation system comprises a computer system configured to drive the first signal and receive the second signal and determine a first updated value for the first potentiometer to obtain a first desired laser output parameter value based on a first known value of the first potentiometer, the second signal, and the first desired laser output parameter value.

Abstract: In one example configuration, an optical package includes a substrate that supports a laser. The laser is configured for electrical communication with circuitry disposed on the substrate, and the laser is arranged to emit an optical signal along a first path. The optical package also includes a beam steering device supported by the substrate and arranged so as to receive the optical signal from the laser along the first path. The beam steering device is configured such that the optical signal is output from the beam steering device along a second path. A group of electronic leads is provided that electronically communicates with the circuitry on the substrate. In this example, the group includes a set of modulation leads in electrical communication with the laser, and a set of bias leads in electrical communication with the laser. The set of bias leads is electrically isolated from the set of modulation leads.

Abstract: A wavelength locker operates with a reference tunable liquid crystal filter having integrated photodetectors deposited on the front and backside of the liquid crystal cell to track power intensity of the accepted passband signal and its rejected signal compliment at a 50% power point where the two signals cross on the side of the filter transmission peak. The tunable filter is tuned by an offset wavelength from the laser wavelength such that 50% power is transmitted through the filter at the center wavelength of the laser. The wavelength locker may be configured with a liquid crystal tunable etalon or a liquid crystal tunable bandpass filter. A method for locking an optical signal to a desired frequency is also included and provides a substantially linear feedback signal computed by dividing the rejected signal power by the accepted signal power and passing it to laser transmitter to enable it to correct for frequency drift.

Abstract: A method for calibrating a laser transmitter includes (a) detecting an eye diagram of an output from the laser transmitter, (b) determining if the eye diagram is acceptable, (c) if the eye diagram is not acceptable, changing a value of a control signal in the laser transmitter, wherein the control signal sets an amplitude characteristic of a limiting amplifier coupled to a laser driver in the laser transmitter, and (d) repeating steps (a), (b), and (c) until the eye diagram is acceptable.

Abstract: A temperature insensitive diode-pumped solid state laser is disclosed in this invention. The components of the laser are optimized with temperature insensitive design, making the laser capable of operating in a wide temperature range without using any cooling/heating system to maintain the temperature of its components.

Abstract: The present invention provides a light emitting device drive circuit which does not require complicated feedback control for shortening a rising response delay time in the transition from an extinction state of a light emitting device to a light emitting state so as not to increase a bias current, and for eliminating harmful effects due to the influence of a droop phenomenon. A circuit including a series of a coil 55 and a resistor 54 is connected in parallel with an LD 53. In this configuration, the LD 53 is subjected to supply of an overshoot drive current ILD for a time period determined by a time constant of the coil 55 and the resistor 54, i.e., a time period in which a high-frequency current is supplied in the transition from the extinction state to the light emitting state. Thus, a delay in rise time until light emission of the LD 53 can be decreased.

Abstract: Systems and methods for regulating optical power are described. A system for regulating optical power includes a laser driver circuit that receives an enable/disable signal and a data modulator input. The enable/disable signal regulates asynchronous mode operation. The system also includes a laser module including a laser diode emitter and a photodiode detector. The laser module is coupled to the laser driver circuit and receives a laser bias current from the laser driver circuit. The system also includes a switch coupled to the photodiode to receive a signal from the photodiode detector. The system also includes an automatic power control (APC) feedback circuit that receives a signal from the switch and provides a laser bias current feedback signal to the laser driver circuit to compensate for power output changes in the laser diode emitter over time.

Abstract: A dental laser arrangement switchable between different modes of operation, the arrangement including a laser device operable as a laser oscillator and as a laser amplifier. The laser device includes a laser resonator having a pump unit and an active modelocker as well as an optic loss element that introduces selective losses in the resonator. The laser arrangement further includes a laser beam-out-coupling unit. The modelocker is connected to a control circuit for switching between a CW-mode of operation for coagulating soft tissue, in which the modelocker is switched off, and two short-pulse modes of operation in which high frequency signals of equal frequencies, but different powers are applied by the control circuit to the modelocker so as to generate laser pulses of a pre-determined duration.

Abstract: A laser transmitter includes an input stage generating an input signal to a limiting amplifier, the limiting amplifier generating an input signal to a laser driver, and the laser driver generating an input signal to a light source. The limiting amplifier has a control terminal for receiving a control signal that sets an amplitude characteristic of the input signal to the laser driver. The amplitude characteristic may be a common-mode or a peak amplitude of the input signal to the laser driver.

Abstract: The invention relates to optical communication devices, light emission devices and optical communication methods. Aspects of the present invention provide methods and devices enable provision of modulated output optical signals without providing modulation of total photon populations of cavities of associated lasers. Such avoids or minimizes bandwidth restrictions associated with conventional methods or devices which modulate total photon populations within cavities of lasers. One aspect of the invention includes an optical communication method including providing a data signal comprising data to be communicated and emitting light using a laser. The method further includes modulating the laser across a lasing mode transition responsive to the data signal. The emitting further includes emitting the light during a plurality of lasing modes of the laser to communicate the data responsive to the modulating.

Abstract: Information of oscillation frequency of a laser beam source in a predetermined range including usable oscillation frequency that takes into consideration an output quality of the laser beam source, such as information on a relationship between oscillation frequency and output quality, is stored in a memory. Then, when the laser beam source is controlled, based on the information stored in the memory, a particular frequency range where the output quality of the laser beam source deteriorates or becomes good is specified, and a decision is made whether or not oscillation frequency f, which is going be used, is within the particular frequency range. Then, according to the results, the oscillation frequency is set, avoiding a frequency range where the output quality deteriorates or within a frequency range where the output quality becomes good according to the result. Thus, a laser beam source whose output quality is always good can be used.

Abstract: Apparatus and methods usable for wavelength references and measurement of wavelength of a light beam. The apparatus comprise at least two wavelength reference or filter elements positioned in a light beam, each wavelength reference element having a different free spectral range and operable to define a joint free spectral range, and a detector positioned in the beam after the wavelength reference elements. The joint free spectral range provided by the multiple wavelength reference elements results in a joint transmission peak that is centered at a unique wavelength, and maximum optical power detectable from the beam by the detector occurs at the center wavelength of the joint transmission peak.

Abstract: A system for measuring the energy of an ultra-short pulse in a laser beam includes a half-wave plate for orienting the polarization of the beam. A polarizing beam splitter is used to reflect a portion of each pulse of the beam and a remainder of the beam is transmitted toward a target. Energy in the reflected portion is measured by a laser energy meter (“LEM”) to determine the energy in the remainder of the beam. An output signal from the LEM is used to obtain an error signal that can then be used to rotate the half-wave plate to control the energy level in the remainder of the beam. In an alternate embodiment, a fixed-ratio beam splitter and a second LEM are used to measure and control the energy in the remainder of the laser beam.

Abstract: All optical clock recovery includes a transmitter for generating an optical timing signal. The transmitter includes a semiconductor laser for the production of a dynamically synchronizable timing signal, the laser having an external resonator for feedback of the timing signal to the laser, the feedback having a delay time greater than a relaxation oscillation time for the laser, and the laser outputting an optical timing signal having a characteristic dynamic. The transmitter supplies the optical timing signal to a receiver configured to receive the timing signal and to synchronize to the laser on receipt of the timing signal, such that the receiver outputs a recovered timing signal having the characteristic dynamic.

Abstract: A high-speed optical transmitter having an optical modulator and a driver circuit while maintaining optical modulator performance. The high-speed optical transmitter comprises the optical modulator which modulates and outputs an input light in accordance with an applied voltage, and a driver circuit which outputs the applied voltage into the optical modulator and has an emitter follower circuit at an output stage thereof.

Abstract: A wavelength locker for use at more than one wavelength includes filters with different characteristics for a corresponding detector. The filters may be etalons having different free spectral ranges, e.g., having different apparent or real thicknesses. If more than three such filters are used outputting offset periodic signals, a reference detector may be eliminated and continuous operation over a wavelength range may be realized.

Abstract: A laser machining apparatus in which laser output can be easily corrected in view of secular change of components of the laser machining apparatus and influence of temperature. Output values of a laser oscillator operated according to a reference laser output command value Psc are stored as reference output values P(?) at different temperatures ? in a table TB1. When the laser oscillator is started up, a measured output value Pr of the laser oscillator supplied with the reference command value Psc is obtained, and a new correction coefficient k is obtained to replace an old one. From a reference output value P(?) at a temperature at that time and the actual output measured value Pr, a correction value ?P is obtained and stored.

Abstract: An optical semiconductor device comprises: an optical semiconductor element; and a circuit. The circuit is connected to the optical semiconductor element and has a series rectifying circuit including a plurality of zener diodes connected in series. The circuit further has a rectifying element whose anode is connected to an anode of the series rectifying circuit.

Abstract: A method for making a laser driver circuit for burst mode is described. A continuous mode laser driver circuit can be used in burst mode transmission system by using the method. The method is described as follows. First, a continuous mode laser driver chip is selected, the chip has a temperature compensation output and a pulse width adjustment input. Next, the temperature compensation signal from the temperature compensation output is converted to a pulse width adjustment signal using an eye diagram adjustment circuit, and the pulse width adjustment signal is sent to the pulse width adjustment input. The pulse width adjustment signal automatically adjusts the cross points of the eye diagram of the laser according to the environmental temperature, so that the cross points are sustained at a predetermined level to reduce the Bit Error Rate.

Abstract: An optical communication system suitable for full duplex optical communication between a first and a second optical transceiver units. connected with each other by a single-core optical fiber. The first and second transceiver units may have different types of light emitting elements emitting light having different loss rates in the single-core optical fiber. The system enables stable full duplex communication without setting up large scale or costly transmission units by relatively reducing the optical crosstalk involved. The first optical transceiver unit provides transmission light to be coupled with the single-core optical fiber with the intensity in the range of TAmin–TAmax. The amount of light emitted from the light emitting element of the optical transceiver unit to be coupled with the single-core optical fiber is in the range TBmin–TBmax, which amount may be adjusted by a light emission adjusting unit provided in the optical transceiver unit.

Abstract: A method for determining a condition of the laser system includes determining a change in a laser current from a preset value. The method includes determining the preset value based on a temperature signal and a supply voltage signal for the laser. The preset value can be determined from a table or from a function.

Abstract: A coherent laser beam combining system wherein the output of a single master oscillator is split into a plurality of N signals, and the N signals are electronically modulated at unique frequencies. There is no reference signal and all of the signals are passed through phase adjusters. All N signals are optically amplified, aligned and passed through a beam splitter to split off a small sample that is imaged onto a photodetector. The photodetector output is fed to a signal processor that separates the N signals and produces N phase error signals that drive the N phase adjusters resulting in a high-powered optically coherent output signal.

Abstract: A method and system for dispersion control of electromagnetic signals in communication networks by narrowing the widths of electromagnetic pulses such as modulated laser signals. Generally, in the preferred embodiment, the present invention utilizes a feedback loop based on dither frequency modulation which dynamically adjusts the alignment of the laser center frequency with the filter passband. In this way, there is an acceptable tradeoff between optical power and pulse width, so a higher power laser can be used to generate a narrower optical pulse. The narrower pulses then travel farther in the fiber link before reaching their dispersion limit. It is believed that, by using this invention, existing link distance could be doubled, while re-using existing installed singlemode fiber. The systems employing the feedback loop may be information carrying or control systems.

Abstract: Embodiments of this invention include a system, method and apparatus for combining light paths of colored light sources through a common integration tunnel.

Abstract: A laser beam is obtained from a semiconductor laser by a stable emission light amount. A first semiconductor laser is thermally coupled with a second semiconductor laser and driven by a feedback circuit constructed by a photodetector, an I-V converter, and a current generator so as to stabilize the emission light amount. A current I0? having a correlation with a drive current I0 of the 1st laser is outputted from a current mirror circuit. A modulation signal is supplied to a current pull-in type current driving circuit via a multiplier and a linearity compensating circuit and a current I2 according to the modulation signal is extracted from a collector of a transistor. The 2nd laser is driven by a current I1 (I0??I2) and a laser beam modulated by the modulation signal is generated. Since the signal modulation by the 2nd laser is performed without influencing on I0 and the 2nd laser is driven by I0?, the emission light amount is stabilized.

Abstract: A laser module for transmitting/receiving digital signals using a laser diode as an optical source is disclosed. The input end of the laser module is provided with a low pass filter, such that the low pass filter makes a 3 dB bandwidth of frequency response transferred from the input pins to the laser diode to be equal or less than the on-state relaxation oscillation frequency of the laser diode, wherein the overshoot is minimized by adjusting a relative amount of the frequency component input to the laser diode.

Abstract: A light source device can attain a stable output of a harmonic even when there occurs a change in the ambient temperature or fluctuation in the output power. The light source device is provided with a semiconductor laser source (4), an optical waveguide-type QPM-SHG device (5) for generating a second harmonic from light emitted from the semiconductor laser source (4), a wavelength control means (7) for controlling a wavelength of light emitted from the semiconductor laser source (4), a means for slightly fluctuating wavelength (8) for changing a wavelength of light emitted from the semiconductor laser source (4) and a means for detecting a change in output light power of the optical waveguide-type QPM-SHG device (5) that occurs when a wavelength of light emitted from the semiconductor laser source (4) is changed.

Abstract: A lighting circuit including a circuit for preventing erroneous illumination of LEDs wherein the lighting circuit, serving as a drive circuit for the LEDs, includes a transistor and a differential amplifier placed at a preceding stage of the transistor. A positive input side of the differential amplifier is connected to a battery through a switch and to a reference resistor Rref. A voltage of the battery is divided by resistors R1 and R2 and supplied to a negative input side of the differential amplifier. A resistance value (R1/R2)×Rref is set to be smaller than a leakage resistance value Rs of the switch. Thus, the differential amplifier is not turned on when the switch is opened even though there is a leakage at the switch. Therefore, the transistor is not turned on, and the LEDs are not illuminated.