Electro-optic sampling apparatus using a low noise pulsed laser diode

Abstract: A low noise pulsed light source using a laser diode (38) for generating a short pulsed light of a high repetitive frequency. The low noise pulsed light source includes a laser diode (38) drived by an electric pulse generator (40) for emitting repetitive pulsed light; a current source for supplying a bias current to the laser diode (38); and a photodetector (42) for detecting the repetitive pulsed light emitted from the laser diode (38); and control means (44). The control means (44) modulates at least one of the bias current from the current source and the amplitude of a pulse signal generated from the electric pulse generator (40) in accordance with an output signal from the photodetector (38) such that the intensity of the pulsed light is kept unchanged and any noise involved in the same is reduced.

Description: The present invention relates to E-O sampling apparatus according to the preamble of claim 1. The apparatus includes a pulsed light source using a laser diode for generating a short pulsed light of a high repetitive frequency (200 to 2 picosecond pulse width, for example), and more specifically to a low noise pulsed light source capable of generating an optical pulse with reduced light intensity noise. An emitted light from a laser diode (LD) changes in its wavelength and intensity as an excitation current and ambient temperature vary. The intensity of the emitted light also changes owing to the competition among longitudinal modes and owing to mode hopping. As a method for reducing such variations of the light intensity, there is known a technique wherein an photodetector element such as a photodiode (PD) detects part of the emitted light from a laser diode to estimate an error signal between a detected light intensity level and a preset one which is in turn fed back to an excitation current source which is to drive the laser diode. Such a technique has already been used for an optical pick-up of a compact disk (CD) player and so on. However, all prior practice to reduce the variations of the light intensity was applied to a laser diode for emitting continuous wave (CW) light or direct current (DC) light. No investigation was paid up to now to the noise involved in the intensity of such pulsed light when it is generated as well as no trial to stabilize the pulsed light intensity. On the other hand, there are many application fields in need of short pulsed light because of temporal resolution being specified by the width of pulsed light. Those fields include an electro optic sampling technique as disclosed in IEEE Journal of Quantum Electronics, Vol. QE-22, No. 1, Jan. 1986, PP 69 through 78 in which an ultrashort light pulse is used as a sampling gate to nondestructively measure an electric signal with use of an electro-optic (E - O) effect; a fluorescence life measuring technique as disclosed in Rev. Sci. Instrum. 59 (4), April 1988, PP 663 through 665 in which an ultrashort light pulse is used to measure laser excited fluorescence; estimation of response characteristics of photoelectric detectors and optical integrated circuits (OE IC), etc.; time correlated photon counting method using a photomultiplier, and so on, for example. A dye laser which generates a picosecond to femtosecond width pulsed light is usable for such applications from the viewpoint of time resolution but with a difficulty of its being large-sized. Instead of this, laser diodes are hopeful as pulsed light sources, because they have some advantages of their being simple and small-sized instructure, inexpensive in manufacture. Now, laser diodes can generate a short pulsed light with an about 200 to 20 picosecond width, and with about 670 nm to 1.5 µm wavelengths in typical, the latter emission wavelengths being varied depending upon the kinds thereof. Additionally, a second harmonic of the pulsed light from a laser diode is available to assure a short wavelength pulsed light up to 340 nm. Repetitive frequencies of such light pulses generally range from 0.1 to 200 MHz although being different in accordance with applications. Furthermore, there are technically available GHz high repetition pulsed light and inversely about several hundred Hz pulsed light. From EP-A-0 218 449, an apparatus for stabilizing the optical output power of a semiconductor laser is known. The laser is supplied with current pulses from a drive circuit. Furthermore, the apparatus includes a current supply which feeds a d.c. bias current to the laser. A monitoring photodiode is used to produce a light detection signal indicative of the actual laser output level. The signal of the monitoring photodiode is supplied to a subtraction device also receiving a pulse signal corresponding to the current pulses. The subtraction device detects the difference between the two signals to produce an error signal. The apparatus further includes integration devices which produce average value signals which are compared by comparators with reference voltages to obtain control signals. The current supply and the current drive circuit are responsive to these control signals to independently modulate the d.c. current and the amplitude of the drive pulses applied to the laser whereby the laser output level can be stabilized to remain a constant level. A voltage detecting device corresponding to the preamble of claim 1 is known from EP-A-0 299 432. The device detects voltages in an object under test and includes an electrooptic material covering a plurality of parts of this object. The refractive index of the electrooptic material is variable according to an applied voltage. A light source emits light through the electrooptic material towards the object and a detecting device receives an emergent light beam reflected from within the electrooptic material in order to detect voltages in the object. It is the object of the present invention to provide an An E-O sampling appartus according to the preamble of claim 1 which is improved over the prior art. This object is achieved by the features indicated in claim 1. In the apparatus having these features, noise is reduced and, in particular, it is possible to realise low noise detection of the whole measuring system by reducing involved noise at a specific frequency. The present inventors have experimentally found that use of a high repetition optical pulse to measurements causes intensity fluctuation of the light pulse to restrict the lower limit of a measurement range, as described below. For simplicity, there will be described the measurement of transmittance of a pulsed light through a sample 10 with use of a device illustrated in Fig. 10. In Fig. 10, a laser diode 12A (refer to Fig. 11) incorporated in a laser diode (LD) pulsed light source 12 emits the optical pulse which is controlled in its repetitive frequency by an oscillator 14 (repetitive frequency 100 MHz, pulse width 50 picosecond, and wavelength 830 nm, for example). The LD pulsed light source 12 is constructed as illustrated in Fig. 11, for example, to which a bias current has previously been supplied and on which a negative pulse is applied from an electric pulse generator 12B (Hewlett Packard, 33002A Comb-Generator (registered trademark) for example) using a step recovery diode for example to drive the LD 12A. The pulsed light emitted from the laser diode (LD) 12A impinges upon the sample 10 through a chopper 16 (chopping frequency 1 kHz, for example) driven by the oscillator 15 and is partly absorbed by and partly transmitted through the same as an output light. The output light is focused by a lens 18 and detected by a photodetector 20 composed of a photodiode (PD) for example. An output signal from the photodetector 20 is amplified by a low noise amplifier 22 and lock-in detected by a lock-in amplifier 24. A chopper signal generated by the oscillator 15 is used for a reference signal in the lock-in amplifier 24. Herein, noises produced in the photodetector 20 and in the low noise amplifier 22 have sufficiently been more reduced than that involved in the transmitted light. An output from the lock-in amplifier 24 is fed to an output meter 26 for example and displayed with respect to the transmittance of the foregoing output light. Herein, although in the foregoing device of Fig. 10 was made of the chopper 16 and of the lock-in amplifier 24 for lock-in detection for the purpose of the reduction of measurement noises and the improvement of measurement accuracy, such construction is unnecessary in simple measurements, in other words, an output from the photodetector 20 may be amplified and read in a direct manner. Further, the low noise amplifier 22 may be omitted and the lock-in amplifier 24 may instead be employed. In such a device, in case where the transmittance of the pulsed light through the sample 10 is nonlinear with respect to the incident pulsed light, the incident pulsed light must be measured up to a sufficiently low level of the intensity thereof. Thereupon, a difficulty is produced of noise involved in the pulsed light emitted from a pulsed oscillation LD, which limits the lower boundary of the measurement. Referring to Fig. 12, exemplary noise characteristics of the LD pulsed light obtained experimentally by the present inventors are illustrated, with the horizontal axis taking frequencies and the vertical axis effective values (rms) of photoelectric current noise in decibel (dB). The point O dB on the vertical axis indicates a short noise level defined by the square root of the number of photons involved in the optical pulse (theoretical limit). Fig. 12 therefore indicates a noise level of the LD pulsed light normalized by the shot noise level. Fig. 12 further illustrates a noise level with the prior system as indicated by a solid line A and marks X. It is understood from the figure that the noise level when the LD undergoes pulsed oscillation is greater 10 times (20 dB) or more than the shot noise level, so that the former may be reduced to the latter shot noise level. The data illustrated in Fig. 12 is given by measuring involved noise produced when the LD 12A to be measured is driven by a driving circuit 30 constructed as illustrated in Fig. 11 using a noise fraction measuring device composed of the photodetector 20, low noise amplifier 22, lock-in amplifier 24, an oscillator (OSC) 32 for frequency sweep, a noise detection circuit 34, and a display 36 as shown in Fig. 13. In the pulsed light source used in the apparatus of the present invention, said control means is a stabilized current modulator circuit that modulates the bias current from the current source in accordance with an output from the photodetector, and the stabilized current modulator circuit comprises, a light intensity signal comparator circuit for comparing the output from the photodetector and a level signal, and making constant a DC component such as a temperature change and temporal drift etc., involved in the light intensity, a light intensity noise extraction circuit for extracting light intensity noise from an output signal of the photodetector to stabilize an AC component such as a ripple that changes at high speed, and a current modulator circuit for modulating a current on the basis of output signals from the light intensity signal comparator circuit and the light intensity noise extraction circuit. More further, in the same pulsed light source, said electric pulse generator comprises a photodetector for detecting part of the pulsed light emitted from the laser diode, and a stabilized electric pulse generator amplituding a pulse signal in accordance with an output signal from the photodetector. Further, in the same pulsed light source, said stabilized electric pulse generator comprises a light intensity signal comparator circuit for comparing a signal from the photodetector with a level signal to make constant a DC component such as a temperature change and temporal drift, etc., involved in the intensity of the pulsed light, a light intensity noise extraction circuit for stabilizing AC components, that change at high speed, such as ripples, etc., and an amplitude modulator circuit for modulating the amplitude of an output pulse voltage on the basis of a signal which has been made constant in DC components thereof by the just-mentioned circuits and from which involved noise has been removed. Further, in the same pulsed light source, said control means modulates the bias current of the current source in a frequency region below a predetermined frequency and modulates the amplitude of a pulse signal generated from the electric pulse generator in a frequency region above the predetermined frequency, in accordance with an output signal from said photodetector such that the intensity of said pulsed light remains unchanged and any noise involved in the same is reduced. Additionally, a step recovery diode is incorporated in said electric pulse generator. Additionally, said photodetector is assembled in the same package together with a laser diode. Fig. 1 illustrates an example of the basic construction of a light source used in the apparatus of the present invention. A laser diode (LD) 38 is first brought to pulsed oscillation by previously supplying a bias current thereto and applying a short pulse electric signal thereto from an electric pulse generator 40 through a capacitor C 1 . An photodetector 42 such as a photodiode (PD) detects splitted light of the LD light or light emanating from the other end of the LD 38. An output from the photodetector 42, which is proportional to the intensity of the LD light is amplified and is used to permit a stabilized current modulator circuit 44 to modulate the bias current of the LD 38 and control the bias current such that the intensity of the LD light remains unchanged. Herein, the time constant of the feedback system is set to be sufficiently longer than the repetitive period of the LD pulsed light. Hereby, the intensity of the LD pulsed light is automatically controlled to remain unchanged, and any noise involved in the LD light is also reduced as indicated by the broken line B and the marks Δ in Fig. 12. As shown in Fig. 2, the foregoing stabilized current modulator circuit 44 comprises, a light intensity signal comparator circuit 44A for comparing the output from the photodetector 42 and a level signal, and making constant a DC component such as a temperature change and temporal drift etc., involved in the light intensity, a light intensity noise extraction circuit 44B for extracting light intensity noise from an output signal of the photodetector 42 to stabilize an AC component such as a ripple that changes at high speed, and a current modulator circuit 44C for modulating a current on the basis of output signals from the light intensity signal comparator circuit 44A and the light intensity noise extraction circuit 44B. Herein, although in the basic construction illustrated in Fig. 1 the bias current supplied to the LD 38 was modulated by the stabilized current modulator circuit 44 over a wide frequency range in accordance with an output signal from the photodetector 42, the construction to make constant the intensity of the LD pulsed light is not limited thereto. For example, the electric pulse generator 40 may be replaced by the photodetector 42 and the stabilized electric pulse generator 41 as illustrated in Fig. 3, and the amplitude of a pulse signal generated by the stabilized electric pulse generator 41 may be modulates in accordance with an output signal from the photodetector 42 via a photoelectric current amplifier 54. Additionally, both may be combined such that the bias current is modulated by the stabilized current modulator circuit 44 in a frequency region below a predetermined frequency while the amplitude of a pulse signal generated by the stabilized electric pulse generator 41 is modulated in a frequency range above the predetermined frequency. With such construction, it is possible to stabilize the intensity of high repetition pulsed light and reduce the light intensity noise. A light pulse generated by such an LD is therefore usable for varieties of measuring fields such for example as E - O sampling, fluorescence lifetime measurement, photoelectric detection, estimation of response characteristics of OE ICs and the like, and temporal correlation photon counting, for the purpose of the improvement of the accuracy of those measurements and of the extension of the lower limits of the measurements. The exact nature of this invention, as well as other objects and advantages thereof will be readily apparent from consideration of the following specification relating to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof and wherein, Fig. 1 is a block diagram exemplarily illustrating the basic construction of a low noise pulsed light source, using a laser diode (LD), used in an apparatus according to the present invention; Fig. 2 is a block diagram illustrating a stabilized current modulator circuit in the above basic construction; Fig. 3 is a block diagram illustraing another basic construction of a low noise pulsed light source used in an apparatus according to the present invention; Fig. 4 is a circuit diagram illustrating the light source of a first embodiment of the present invention; Fig. 5 is a circuit block diagram exemplarily illustrating the concrete construction of the light source of the first embodiment of the present invention; Fig. 6 is a block diagram illustrating the light source of a second embodiment of the present invention; Fig. 7 is a block diagram illustrating the light source of a third embodiment of the present invention: Fig. 8 is a diagram illustrating the principle of a light source of a fourth embodiment of the present invention; Fig. 9 is a block diagram exemplarily illustrating the construction of an E - O sampling device according to the fourth embodiment; Fig. 10 is a block diagram exemplarily illustrating the construction of a transmittance measuring device intended to describe the difficulties of the prior art; Fig. 11 is a block diagram exemplarily illustrating the construction of an LD pulsed light source used in the device of Fig. 10; Fig. 12 is a diagram illustrating a comparison between frequency characteristics of noise levels; and Fig. 13 is a block diagram exemplarily illustrating a noise component measuring device used for obtaining the data in Fig. 12. In what follows, embodiments of the present invention will be described with reference to the accompanying drawings. A first embodiment of the present invention includes the basic construction illustrated in Fig. 1 and principally comprises, as illustrated in Fig. 4, a reverse-biassed pin photodiode (PIN - PD) 43 for detecting part of emitted light from the LD 38, and a current modulator circuit 44 composed of a first amplifier 46 for inversely amplifying a photoelectric current signal yielded by the PIN - PD 43 to alter the DC level of an output signal and of a second amplifier 48 as an inverted current amplifier for inverting and amplifying the output of the first amplifier 46. The first amplifier 46 has a feedback loop that incorporates a capacitor C 2 for making longer the time constant of the first amplifier 46 than the repetitive period of the pulsed light. Additionally, the second amplifier 48 has a feedback loop that incorporates a diode D for making zero the minimum of the bias current to prevent an opposite bias current from flowing. Other features are identical to those in the basic construction illustrated in Fig. 1 and the description thereof will be omitted. Operation of the first embodiment is as follows. As the intensity of pulsed light from the LD 38 is increased, for example, a photoelectric current from the PIN - PD 43 is increased to raise the potential at the point a in the figure. Hereby, the output voltage of the first amplifier 46 is decreased, and hence the output current of the second amplifier 48 (the direction of the arrow in the figure is assumed to be positive) or the bias current is decreased. Therefore, the intensity of the output light from the LD 38 is reduced and controlled such that it remains unchanged. Referring to Fig. 5, the concrete construction of the light source of the first embodiment is exemplarily illustrated. In the present example, the amplifiers 46, 48 comprise transistors, capacitors, and resistors, etc., respectively. Further, a set of the LD 38 and the PIN - PD 38 is housed in the same package 50, for miniaturization thereof. In succession, a light source according to a second embodiment of the present invention will be described with reference to Fig. 6. In the second embodiment, a direct current (DC) bias current supplied from a current source 52 to the LD 38 is made constant, while a signal detected by the photodetector 42 is amplified by the photoelectric current amplifier 54 and adapted to modulate the amplitude of a pulse voltage applied from the stabilized electric pulse generator 41, that is controllable in amplitude over a wide frequency range, to the LD 38. The stabilized electric pulse generator 41 comprises a light intensity signal comparator circuit 41A for comparing a signal from the amplifier 54 with a level signal to make constant a DC component such as a temperature change and temporal drift, etc., involved in the intensity of the LD pulsed light, a light intensity noise extraction circuit 41B for stabilizing AC components, that change at high speed, such as ripples, etc., and an amplitude modulator circuit 41C for modulating the amplitude of an output pulse voltage on the basis of a signal which has been made constant in DC components thereof by the just-mentioned circuits and from which involved noise has been removed. More specifically, when the intensity of the LD pulsed light is strong, the amplitude of the pulsed voltage is controlled such that it is reduced. Hereby, the intensity of the LD pulsed light is kept unchanged and any involved noise is reduced. Herein, although in the first and second embodiments any one of the amplitudes of the DC bias current and the pulse voltage was modulated, both may be done simultaneously, as disclosed in a third embodiment illustrated in Fig. 7. In the third embodiment, it is effective to separate the modulation frequency regions of the two modulation systems by a controller 55. For example, a frequency range of from DC to 1 kHz may be controlled by the DC bias current while a frequency range above 1 kHz may be controlled by the amplitude of the pulse voltage. Successively, a light source according to a fourth embodiment of the present invention will be described. In the fourth embodiment, frequency characteristics are provided to the feedback system (optical detection → amplification → control) described in the first and second embodiments, and in normalized noise obtained in such a manner and illustrated in Fig. 8 a lock-in amplifier in a measuring system is locked at a frequency where less noise is involved. To be concrete, Fig. 9 exemplarily illustrates the complete construction of the fourth embodiment. In this construction example, laser light emitted from the LD 38 (pulsed light for sampling) is fed to an optical probe 64 disposed upwardly of an IC 62 to be measured through an optical fiber 60 for example. The optical probe 64 comprises a lens 64A, a half mirror 64B and an electrooptic crystal 64C, and is adapted to modulate the pulsed light for sampling by making use of a change in the refractive index of the electrooptic crystal 64C caused by an electric field induced on the surface of the IC 62 by a current flowing through the same. The pulsed light modulated by the change in the refractive index is detected by the photodetector 66 and outputted through the lock-in amplifier 68. The lock-in amplifier 68 and an IC driver circuit 80, which switches an electric circuit on and off at a lock-in frequency, are controlled by an output from an oscillator 70 for switching an electric pulse on and off and outputs a synchronizing signal to the electric pulse generator 40. Such construction assures noise reduction. Additionally, although it is generally difficult to assure low noise detection over a wide frequency range, it is possible to realize low noise detection of the whole measuring system by reducing involved noise at a specific frequency.

Claims: A non destructive electro-optic (E-O) sampling apparatus, comprising a pulsed light source and an optical probe (64) disposed near an object (62) to be measured, the optical probe (64) includes an electrooptic crystal and the light pulse for sampling is modulated by making use of a change in the refractive index of the electro optic crystal caused by an electric field induced on the surface of the object (62) by a current flowing through the same, characterised in that the pulsed light source is a low noise pulsed light source including a laser diode (38) for emitting repetitive pulsed light, an electric pulse generator (40) for driving the laser diode (38), a current source for supplying a bias current to the laser diode (38), a photodetector (42) for detecting the repetitive pulsed light emitted from the laser diode (38), and a control means for modulating at least the bias current from the current source and the amplitude of a pulse signal generated from the electric pulse generator (40) in accordance with an output signal from the photodetector (42) thereby forming a feedback system such that the intensity of the pulsed light is kept unchanged; wherein the feedback system has a time constant longer than the repetitive period of the pulsed light and a peak in its frequency characteristics to reduce any noise involved in a specific frequency range, and the pulsed light modulated by the change in the refractive index is detected by a further photodetector (66) and outputted through a lock-in amplifier (68) driven at a lock-in frequency falling within the specific frequency range. An E-O sampling apparatus according to claim 1 characterised in that the control means is a stabilized current modulator circuit (44) that modulates the bias current from the current source in accordance with an output from the photodetector (42), and the stabilized current modulator circuit (44) comprises, a light intensity signal comparator circuit (44A) for comparing the output from the photodetector (42) and a level signal, and making constant a DC component such as a temperature change and temporal drift etc., involved in the light intensity, a light intensity noise extraction circuit (44B) for extracting light intensity noise from an output signal of the photodetector (42) to stabilize an AC component such as a ripple that changes at high speed, and a current modulator circuit (44C) for modulating a current on the basis of output signals from the light intensity signal comparator circuit (44A) and the light intensity noise extraction circuit (44B). An E-O sampling apparatus according to claim 1 characterised in that the electric pulse generator comprises a photodetector (42) for detecting part of the pulsed light emitted from the laser diode (38), and a stabilized electric pulse generator (41) amplituding a pulse signal in accordance with an output signal from the photodetector (42). An E-O sampling apparatus according to claim 3 characterised in that the stabilized electric pulse generator (41) comprises a light intensity signal comparator circuit (41A) for comparing a signal from the photodetector (42) with a level signal to make constant a DC component such as a temperature change and temporal drift, etc., involved in the intensity of the pulsed light, a light intensity noise extraction circuit (41B) for stabilizing AC components, that change at high speed, such as ripples, etc., and an amplitude modulator circuit (41C) for modulating the amplitude of an output pulse voltage on the basis of a signal which has been made constant in DC components thereof by the just-mentioned circuits and from which involved noise has been removed. An E-O sampling apparatus according to claim 1 characterised in that the control means (44,54) modulates the bias current from the current source in a frequency range below a predetermined frequency and modulates the amplitude of a pulse signal generated from the electric pulse generator (40) in a frequency range above the predetermined frequency, in accordance with an output signal from the photodetector (42) such that the intensity of the pulsed light is kept unchanged and any noise involved in the same is reduced. An E-O sampling apparatus according to claim 1 characterised in that the electric pulse generator (40,41) is of the type using a step recovery diode. An E-O sampling apparatus according to claim 1 characterised in that the photodetector (43) is assembled as a set with the laser diode (38) into the same package (50).

Inventors: NAKAMURA TAKUYA, TAKAHASHI HIRONORI, TSUCHIYA YUTAKA, NAKAMURA, TAKUYA, TAKAHASHI, HIRONORI, TSUCHIYA, YUTAKA
Applicants: HAMAMATSU PHOTONICS KK, HAMAMATSU PHOTONICS K.K.
Application year: 1990
Main classification: H01S3
IPC codes: G01R15, H01S5