GB2224352A - Light waveform measuring apparatus - Google Patents

Abstract

A semiconductor laser (1) as a pulse light source emits main laser light to irradiate a sample (3) and also omits monitor laser light which is in a direction opposite to that of the main laser light and in synchronism with the main laser light. The monitor laser light is detected by a photodetector (4). A waveform of light (11) of interest emitted from the sample is measured with measuring moans (8) on the basis of an output signal from the photodetector (6). The laser used comprises a package having a window (23) through which the laser light is passed and a photodetector (4) for receiving monitor laser light (10) from laser (1) mounted on projection (21). The semiconductor laser is supplied with a current pulse via positive lead (24) and negative lead (25) and the photodetector output is picked up via positive lead (24) and negative lead (26). <IMAGE>

Description

LIGHT WAVEFORM MEASURING APPARATUS BACKGROUND OF THE INVENTION The present invention relates to an apparatus for measuring the waveform of light emitted from a sample upon exposure to a laser beam.

While various types of apparatus are available for measuring the waveform of light emitted from a sample upon exposure to a laser beam, it is known that the life of fluorescence can be measured by a time-correlation-singlephoton-counting method. To briefly describe this time correlation-single-photon-countingvmethod, a light pulse of a satisfactorily short duration is applied to the sample and the fluorescence photon emitted from the sample upon exposure is detected, thereby measuring the time from the application of light pulse to the detection of fluorescence photon. The photon detection is so adjusted that no more than a single photon is detected upon one application of light pulse.This time measurement is repeated a number of times (as many as a million times if precise results are required) and the many sets of data obtained are represented in a histogram to determine the life characteristics of fluorescence from the sample (light waveform).

For successful performance of the time-correlationsingle-photon-counting method, it is necessary that the duration of time from the application of a light pulse to the sample to the detection of a fluorescence photon be measured correctly. To meet this requirement, a measurement start signal which serves as a reference for starting the time measurement must be obtained for each application of optical pulse. This situation is not limited to the case of using the time-correlation-single-photon-counting method but applies commonly to every case of measuring the waveform of light emitted from a sample upon exposure to laser light.

The measurement start signal has conventionally obtained by the following two methods. The first method comprises splitting an optical pulse with suitable means such as a half mirror and using one beamlet to excite the sample, with the other beamlet being detected with a suitable device such as a photodetector to produce a detection output which is used as the measurement start signal. In the second method, an electric pulse for driving the pulse light source is directly used as the measurement start signal.

In the first method which uses a half mirror, one of the beamlets obtained by splitting with the mirror must be focused in alignment with the optical axis of the photodetector and the need for this optical setup complicates the composition of equipment which hence becomes bulky and expensive. A further problem with this approach is that the half mirror inserted between the sample and the light source attenuates the optical pulse being applied to the sample.

The second method in which an electric signal for driving the pulse light source is directly used as the measurement start signal also has the disadvantage that drifts will occur on account of such factors as changes in the temperature of the drive circuit and pulse light source, making it impossible to maintain a constant time interval between the actual application of a light pulse and the issuance of the measurement start signal. Because of this problem, the second method has been unsuitable for measurements at the high time resolution.

SUMMARY OF THE INVENTION An object, therefore, of the present invention is to solve the aforementioned problems of the prior art. This object of the present invention can be attained by a light waveform measuring apparatus which comprises a semiconductor laser as an pulse light source, a first photodetector for detecting a monitor laser beam that is emitted from said semiconductor laser in opposite direction to the main laser beam emitted therefrom to be launched into a sample, and measuring means for measuring the waveform of light of interest from the sample on the basis of the output from said first photodetector.

In the apparatus having the construction described above, the monitor laser beam emitted from the semiconductor laser in synchronism with the main laser beam is detected and the waveform of light of interest is measured on the basis of the resulting detection output.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram of a light waveform measuring apparatus according to an embodiment of the present inention; Fig. 2 is a cross-sectional view showing how a semiconductor laser is installed relative to a first photodetector; Fig. 3 is a graph showing the results of computation with PHA; Fig. 4 is a cross-sectional view showing another arrangement of installation of the semiconductor laser relative to the first photodetector; Fig. 5 is a block diagram of a light waveform measuring apparatus according to another embodiment of the present invention; Fig. 6 is a schematic view showing the construction of a streak camera; Fig. 7 is a schematic view showing the construction of light waveform observing means of sampling type; and Fig. 8 is a diagram for explaining how a light waveform is obtained with the light waveform observing means of sampling type.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1 is a block diagram showing a light waveform measuring apparatus according to an embodiment of the present invention. A semiconductor laser (laser diode) 1 is excited by a rapid rising current pulse from a pulse source 2 to produce pulses of a satisfactorily short duration ( < 1 ns). On an extension of the axis of the main laser beam emitted from the semiconductor laser 1, a sample 3 is located and irradiated with a light pulse 9 excited from the semiconductor laser 1.

A monitor light pulse 10 is also emitted from the semiconductor 1 but in opposite direction to the main beam and a first photodetector 4 for detecting this monitor light pulse is located on an extension of its axis. This photodetector 4 may be composed of a photodiode and incorporated in the package of semiconductor laser 1.

Fig. 2 is a cross-sectional view showing how the photodiode 4 may be encased in the package of the semiconductor laser 1. The semiconductor laser 1 is located at the front end of a projection 21 from the central portion of a stem 20, and the photodiode serving as the first photodetector 4 is located at the base end of the projection 21. The stem 20 is provided with a cap 22 in such a way as to enclose the semiconductor laser 1 and photodetector 4. The main beam 9 from the semiconductor laser 1 is emitted through a glass window 23 provided in the center of the cap 22. The monitor light pulse 10 emitted in the opposite direction to the main beam is launched into the photodetector 4. The semiconductor laser 1 is supplied with a current pulse via a positive lead 24 and a negative lead 25. The output from the photodetector 4 is picked up via the positive lead 24 and a negative lead 26.

The output of the first photodetector 4 is fed as a measurement start signal to the start terminal of a time-toamplitude converter (TAC) 5 as shown in Fig. 1.

When the light pulse 9 emitted from the semiconductor laser 1 in the main beam direction is launched into the sample 3, the latter will emit fluorescence photons 11 which are detected by a second photodetector 6 typically made of a photomultiplier tube. The detection output of this photodetector 6 is fed to the STOP terminal of TAC 5. The photodetector 6 is set for such a condition that it will detect one photon 11 only after the sample 3 has been subjected to application of several tens of light pulses.

TAC 5 is a device that outputs a voltage pulse having a height proportional to the time difference between the signal fed at the START terminal and the signal fed at the STOP terminal. Thus, aside from the actual lag due to the propagation times of laser light and the detection output, the output voltage pulse has a height corresponding to the time between the application of light pulse from the semiconductor laser 1 and the emission of a fluorescence photon from the sample 3. As already mentioned, the second photodetector 6 is set to operate under such a condition that it detects one photon 11 upon application of several tens of optical pulses, so TAC 5 will produce its output voltage pulse only once when it receives several tens of measurement start signals.

The output terminal of TAC 5 is connected to the input terminal of a pulse height analyzer (PHA) 7. PHA 7 stores the height of each input voltage pulse in a digitized form and counts the number of input pulses according to their height. This analyzer combines with TAC 5 to make up means 8 for computing the life characteristics of fluorescence.

Fig. 3 is a graph illustrating the results of counting which is conducted with PHA 7 when it receives multiple voltage pulses from TAC 5. The horizontal axis of the graph plots the time in proportion to the height of voltage pulse and the vertical axis plots the number of voltage pulses.

The vertical axis shows the probability that a photon 11 will be detected at a certain time and its value is proportional to the intensity of fluorescent light at that time. Hence, the graph is a direct presentation of the time profile of intensity, of fluorescent light, or life characteristics of fluorescence. In practice, from a million to several tens of millions of optical pulses are applied and measurements are continued until about a million photons have been detected and the life characteristics of fluorescence are computed on the basis of the results of measurements.

In the embodiment shown in Fig. 1, both the semiconductor laser 1 and the first photodetector 4 are encased within the same package. Another layout for the arrangement of semiconductor laser 1 and the first photodetector 4 is shown in Fig. 4. A glass window 40 is also formed in the stem 20 and one end of an optical fiber is placed in contact with this glass window 40, with the other end being connected to the external photodetector 4 (not shown) to which the monitor light pulse 10 is guided through the optical fiber. In this case too, the main laser beam 9 is to be used only for the purpose of irradiating the sample 3.

Alternatively, the monitor light pulse 10 transmitting the glass window 40 may simply propagate through air, rather than the optical fiber, to be guided to the photodetector 4. If necessary, the photodetector 4 may be directly attached to the glass window 40.

Fig. 5 is a block diagram of a light waveform measuring apparatus according to another embodiment of the present invention. The components which are the same as those shown in Fig. 1 are identified by like numerals and will not be described in detail. In this embodiment, a streak camera 50 is used in place of TAC 5, PHA 7 and photodetector 6. The construction of this streak camera and its operation need be described briefly with reference to Fig. 6. As shown, the streak camera generally indicated by 50 consists basically of a streak tube 52 and a camera 53 taking the streak image produced with the streak tube 52. Light 51 that is emitted from the sample and is to be measured with the streak camera passes through an input optical system including a slit (not shown) and lenses (also not shown) and reaches a photocathode 55 of the streak tube 52, where it is converted to electrons.

The resulting photoelectrons are accelerated by means of an accelerating electrode 56 and guided between deflecting plated 57 toward a microchannel plate (MCP) 58. As they pass between the deflecting plates 57, the photoelectrons are swept by the sweep voltage applied between the deflecting plates 57 and arrive at the MCP 58. The photoelectrons are multiplied at the MCP 58 by a sufficient factor and excite a phosphor screen 60 for forming a streak image. The streak image is taken with the camera 53 placed behind the phosphor screen 60. The sweep voltage must be applied to the deflecting plates 57 in synchronism with the passage of photoelectrons between those plates.To this end, the streak camera 50 is supplied with the detection output of the first photodetector 4 as a trigger signal for starting the sweep operation and in response to this trigger input, a sweep voltage generator 62 generates the sweep voltage which is applied to the deflecting plates 57.

When the light pulse 9 in the main beam direction from the semiconductor laser 1 is launched into the sample 3, the latter emits light 51 to be measured. This light 51 is launched into the streak camera 50 which produces a streak image that visualizes the waveform, or temporal change of intensity, of the light 51 emitted from the sample 3.

The measurement described above is repeated and the resulting streak images are integrated with a suitable apparatus such as an image processor (not shown) to reconstruct a streak image having a high signal-to-noise-ratio (S/N ratio), or the accurate waveform of the light of interest.

The streak camera 50 shown in Figs. 5 and 6 may be replaced by light waveform observing means of sampling type.

An exemplary construction of this light waveform observing means of sampling type and its operation are briefly described with reference to Fig. 7. The apparatus shown in Fig. 7 consists basically of a sampling type streak tube 65 and an information processor 66 which processes the information on the waveform of light 51 obtained by extracting part of it with the streak tube. The light 51 emitted from the sample 3 and to be observed with the apparatus is focused with a lens 67 on a photocathode 68 of the streak tube 65. The incident light on the photocathode 68 is converted to electrons the number of which is proportional to the intensity of light. The resulting photoelectrons are accelerated by an accelerating electrode 70 and guided between deflecting plates 71 to reach a slit plate 72.As they pass between the deflecting plates 71, the photoelectrons are swept by the sweep voltage applied between the plates 71 and arrive at the slit plate 72. Since a tiny slit perpendicular to the sweep direction is formed in the plate 72, only part of the electrons can pass through the plate to reach a phosphor screen 73 behind it. The phosphor screen 73 emits light upon excitation by the impinging electrons. The intensity of emitted light is sensed by a photomultiplier tube 75 and amplified by an amplifier. 76 to produce an output electric signal. The signal thus obtained by sampling the intensity of light 51 is stored in the information processor 66.

The sampling operation described above is repeated with the timing of each sweep being delayed by small but increasing amounts from the incidence of light 51 and the information obtained is processed to obtain a light waveform as depicted in Fig. 8.

In the apparatus shown in Fig. 7, the detection output of the first photodetector 4 is fed to the sampling type streak tube 65 and each application of sweep voltage to the deflecting plates 71 is delayed progressively from the time at which the detection output of the first photodetector 4 is fed to the streak tube 65.

In the embodiments described above, the main laser beam 9 emitted from the semiconductor laser 1 is directly launched into the sample 3. If desired, wavelength converting means (not shown) may be provided between the semiconductor laser 1 and the sample 3 so that a laser beam having a different wavelength than the laser beam from the semiconductor laser 1 (for example, a laser beam having one half of the wavelength of the latter) is launched into the sample 3. Such wavelength converting means may be formed of an optical crystalline material having a nonlinear optical effect as typified by lithium niobate (LiNbO3).

As described on the foregoing pages, the light waveform measuring apparatus of the present invention detects the monitor light beam from the semiconductor laser and measures the waveform of light of interest on the basis of the resulting detection output. Hence, the apparatus of the present invention is capable of precisely obtaining irradiation times of light pulses. Further, no optical device such as beam splitting means that requires complicated procedures in setting up need to be disposed between the semiconductor and the sample, and this contributes to the fabrication of a compact and inexpensive system which yet is capable of measuring the waveform of light of interest with high precision.

Claims (9)

WHAT IS CLAIMED IS:

1. A light waveform measuring apparatus for measuring light of interest that is emitted from a sample upon exposure to laser light, comprising: a semiconductor laser for emitting main laser light to irradiate said sample and monitor laser light in a direction opposite to a direction of said main laser light; a first photodetector for detecting said monitor laser light; and measuring means for detecting said light of interest from said sample to measure a waveform thereof on the basis of an output signal from said first photodetector.

2. The apparatus according to claim 1, wherein said first photodetector is encased within a package of said semiconductor laser.

3. The apparatus according to claim 1, wherein said semiconductor laser is encased within a package having a window through which said monitor laser light is to be passed, and transmitted monitor laser light from said window is guided to said first photodetector.

4. The apparatus according to claim 3, wherein said window is connected to said first photodetector with an optical fiber.

5. The apparatus according to claim 3, wherein said first photodetector is directly attached to said window.

6. The apparatus according to any one of claims 1, 2, 3, 4 and 5, wherein said measuring means comprises a streak camera which performs its sweeping operation on the basis of said output signal from said first photodetector.

7. The apparatus according to any one of claims 1, 2, 3, 4 and 5, wherein said measuring means comprises light observing means of sampling type which operates on the basis of said output signal from said first photodetector.

8. The apparatus according to any one of claims 1, 2, 3, 4 and 5, wherein said measuring means comprises: a second photodetector for detecting photons of said light of interest from said sample; and means for performing a plurality of measurements of a time between an output from said first photodetector and an output from said second photodetector, and computing said waveform of said light of interest by a time-correlationsingle-photon-counting method.

9. An apparatus substantially as described with respect to the accompanying drawings.