Here we explain about measuring instruments.
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Table of Contents
1.2. Beam Profiler
1.3. Wavefront Sensor
For the measurement of an ultrashort light pulse it is necessary to consider various light source’s characteristics.
It is mandatory to select the measuring method taking into consideration such aspects as: dispersion by the propagation medium, pulse width, band width, spectrum width, coherence, polarization characteristics, instantaneous intensity, integrated intensity and repetition period. Ultrafast phenomena such as ultrashort light pulse cannot be directly observed via oscilloscope by capturing it with a photodetector. Because of this, it must be indirectly measured by measuring a phenomenon that can be observed. Explained here is dispersion as a light-pulse-shape changing factor and specific measuring methods.
When a light pulse with a wide spectrum width propagates through a medium, the light-pulse shape changes due to the wavelength dependence of the refractive index.
This is due to the difference in the propagation time of each frequency component in the light pulse (Group delay) and each frequency component deviating from the light pulse’s center of gravity (GVD: group velocity dispersion). The wider the spectrum width, the more group velocity dispersion takes effect. For example a 10 fs light pulse with an 800 nm center wavelength has a 100 nm spectrum width. With such light pulses the pulse width becomes 80 fs just by propagating 10 m in the atmosphere. It is needed to take into account the effect of dispersion when measuring ultrashort light pulses and also designing the optical system of the measuring device.
It is effective to consider the dispersion of the group refractive index when analyzing such a light pulse. Bellow shows group refractive index ng.
The n0 and λ here each represent refractive index and wavelength in vacuum.
Refractive index of the material n0 uses Sellmeier equation or the following equation which is a Drude model approximation.
A and λ0 are obtained by the fitting when approximating the refractive index using Eq. 5.1.2. Equation 5.1.3 shows a good match in the wavelength region under 1 μm of a transparent medium that shows absorption only in the ultraviolet region. The light pulse’s width broadening over time can be calculated using GVD and the refractive index obtained by Eq. 5.1.1 and 5.1.2.
When considering the case of the Gaussian pulse with the pulse-width of tin propagating through a dispersion medium, the output light pulse width tout can be obtained by the following equation.
The L is the distance that the light pulse has propagated through the dispersion medium. B is a coefficient obtained by the following approximate equation.
Table 5.1.1. Relationship between wavelength and the value of B of various mediums
As known measuring methods of ultrashort light pulses there are: i) A method using a high speed photoelectric conversion device, ii) A method that measures the light pulse’s cross-correlation waveform and autocorrelation, iii) A method using interference, iv) A method using a streak camera. The following will explain each measuring method. Also table 2 shows the characteristics of each method.
Table 5.1.2. Various ultrashort light pulse measuring method’s characteristics
|Method||Wavelength region [nm]||Time response||Repetition|
|Photoelectric conversion||Baipurana phototube||115 – 1100||60 ps||Single – GHz|
|Photomultiplier tube||115 – 1700||150 ps||Single – GHz|
|PIN-PD||350 – 1600||250 ps||Single – GHz|
|APD||350 – 1600||300 ps||Single – GHz|
|Correlation measurement||Autocorrelation method||Ultraviolet – Infrared||A few fs||～ A few 100 MHz|
|FROG||Ultraviolet – Infrared||A few fs||～ A few 100 MHz|
|Upconversion||Visible – Far infrared||A few fs||～ A few 100 MHz|
|Time correlated single photon counting method||115 – 1600||20 ps||～200 kHz|
|Interference measurement||Homodyne method||Optional||A few fs||～ A few 100 MHz|
|Streak camera||X ray – 1600||200 fs||Single – GHz|
a. Method using a high speed photoelectric conversion device
The simplest method of measuring the waveform of a light pulse is the method that directly measures it using a photodetector capable of high-speed photoelectric conversion and a sampling oscilloscope. As for photodetector there are phototube equipped with a photoelectric conversion device in a vacuum tube, semiconductor device and others. Figure 1 shows the measuring method’s conceptual diagram.
The light pulse from the light source is guided to the high-speed photoelectric conversion device after which electrical signal that is photo-electrically converted is intercepted by the oscilloscope. Time resolution of this measurement system is determined by a variety of factors such as: impulse response of the fast photoelectric conversion device, sampling pulse width of the oscilloscope, clock stability of sampling and trigger jitter of the synchronization system. However, it is mostly determined by the response time of the fast photoelectric conversion device. Because the time resolution is a few hundred ps, waveform measurement is possible of an optical pulse of ns.
Fig. 5.1.1. Method using the photoelectric conversion device.
There are many types of phototubes. The differences are in the photoelectric conversion material, electron – electrical signal conversion mechanism, and electron-multiplier mechanism.
Phototubes used for visible range uses photoelectric surfaces which are called S1, S20 or S25 (Standard of photoelectric surface sensitivity of the U. S.) which are a composite of alkali metal. Time response of phototubes is determined by the processing system after light conversion into electrons.
A far infrared phototube uses semiconductors (such as GaAs and InGaAs) as photoelectric surface. Since the semiconductors have a much higher sensitivity than a S1 photoelectric surface up till 1.7 μm, thus it is used for the time resolved measurements of the far infrared region even though the response time is slightly slow. For the ultraviolet region a phototube which has a diamond, which has a high incident photon-to-current conversion efficiency, as a photoelectric surface.
Phototubes that do not have an electronic multiplication mechanism are called biplanar phototube. On the other hand, phototubes that have an electronic multiplication mechanism called dynode or micro-channel plate (MCP) are called electron multipliers (referred to as PMT or photomultiplier), and time resolution is about several hundred ps .
The latter amplifies the electronic signal by applying a high voltage between the electrodes to apply secondary electron multiplying over a multistage to the photoelectrons inputted into the inner glass tube. Because of this it is used for detecting very weak light.
Those using semiconductors as photoelectric conversion device are called photodiode (PD). Semiconductor PDs have a high photoelectron conversion efficiency. They are easier to use compared to photomultiplier tubes because constant voltage operation is possible. They are also small and inexpensive. Various semiconductors are used on each wavelength. From ultra violet region to near infrared region Si or InGaAs are used. Other examples are: Ge that has a sensitivity making it possible to detect up to 1.8 μm and HgCdTe that is cooled to 77 K using liquid nitrogen that has a sensitivity making it possible to detect up to 17 μm.
Semiconductor PDs are divided in to MSM structure and PIN structure and are farther divided in to such things as devices for avalanche operations.
Ones that takes a PIN structure formed by joining a p-type semiconductor and an n-type semiconductor are common. It takes out the optical signal as a current through the generation of holes – electrons generated at the bonding surface when irradiated with light. In the case of an avalanche photodiode (APD) it causes a secondary electron multiplication by causing an electron avalanche with electron current generated by light as a result of applying a high voltage of approximately 200 V inside the semiconductor.
This enables it to have a time response under 300 ps while still having the gain of an electron multiplication.
Detectors incorporating the advantages of both semiconductor APD and photomultiplier tube have also been developed. It is possible to obtain a multiplication gain of the electrons of over 104. Although the response time is ns order it is possible of single photoelectron detection and also there is little photomultiplier fluctuation. Because of this it is possible of detecting each photoelectrons of more than two and is suitable for the detection of a weak high speed phenomena.
b. Method of Measuring Correlation Waveform
・Frequency-resolved optical gating (FROG)
This method detects cross-correlation signal by generating a sum-frequency of the optical pulse which needs to be measured and the reference optical pulse. By choosing the reference optical pulse correctly it is possible to observe a phenomenon or ultrashort –optical-pulse waveform of a long-wavelength band that the detector originally does not have the sensitivity, with high sensitivity. In order to measure it is needed to generate a sum frequency by simultaneously focusing the long-wavelength light which needs to be measured and a short-length light pulse (such as e.g. 800 nm) for reference into a nonlinear optical crystal. Measurement is achieved by measuring the shape of the long-wavelength-light pulse by changing the delay of the reference light and measuring the sum frequency. In an actual measurement it is mandatory to choose an appropriate non-linear optical crystal for sum-frequency generation and set the crystal axis, angle of incidence, polarization and crystal temperature properly so that the phase matching conditions are satisfied.
・Time-correlated single photon counting method
This method makes it possible to measure a waveform of an optical pulse from a weak light source with a high dynamic range. Figure 5.1.2 shows the schematic diagram of the measurement system. This figure shows the measuring of the time response of an optical phenomenon induced by a pulsed light. This measurement system uses a single-photon detection type photomultiplier tube and a time-to-amplitude convertor called TAC. TAC is a device that converts the arrival time interval of two electrical signals into voltage. The light pulse from a light source is divided into two. One generates the START signal of the TAC by light detection. The other is used to generate some sort of optical phenomena. The optical phenomenon that occurs is detected after the amount of light is dropped to single-photon level by the ND filter. An amplitude discriminator is used to distinguish between an electrical signal when a single photon is detected and an incident of an electrical signal of multiple photons or thermal noise of the circuit. The electric signal generated from a single photon is used as a STOP signal for TAC. By varying the timing of the START signal using a delay circuit it is possible to measure the optical waveform. This is the amplitude distribution corresponding to the probability of the photons from the light source.
Fig. 5.1.2. Time-correlated single photon counting technique．
・Methods Using Interference
By introducing a beam with a stable phase to the interferometer, a stable interference fringes can be obtained. From this it is possible to know the characteristics of the incident light. It is possible to determine the duration of coherent light pulses from the relation of the amount of delay and interference fringes obtainable from introducing the light pulse into an interferometer with a tunable delay amount. Note that with this method only measures the coherent time width corresponding to the spectral width of the optical pulse. Meaning it is not possible to obtain the effect of group velocity dispersion in the propagation process or light source.
Figure 5.1.3 shows a measurement system schematic diagram of a time-domain balanced homodyne for measuring time width by using an optical interference. Light pulse is introduced into the Mach-Zehnder interferometer and output lights are each detected by the photodetector. Mach-Zehnder interferometer has two optical outputs. From each output a complementary interference pattern can be obtained. Electrical signal obtained from the photodetector is differentially amplified and the only interference component is obtained and then analyzed by an oscilloscope. With balance detection the influence of the intensity fluctuations of the light source can be removed at the same time (Common-mode suppression). By measuring the interference pattern by changing the amount of delay coherent time width is measured.
As for methods to improve the sensitivity there are: AC balanced homodyne method of alternating current modulating the delay and performing a lock-in detection by inserting a chopper in the light path.
Fig. 5.1.3. Time-domain balanced homodyne method．
・Method using a streak camera
Streak camera is a measuring device having a time resolution of about 200 fs and can analyze single photon level information that is not the time dependent. It is effective for direct observation of the dynamics of the light source. Figure 5.1.4 shows a schematic diagram of a measurement system using a streak camera. Through the photoelectric surface, the light pulse is converted to an optoelectronic image which has the same intensity waveform. The photoelectrons are accelerated by a voltage and introduced to a pair of parallel plates. At this time, if apply a lamp sweeping voltage synchronized with the light emission timing of the light source to the parallel plates, the photoelectrons will be sequentially deflected from top to bottom depending on the voltage. In other words, time-domain waveform of the photoelectrons is converted into a spatial intensity distribution in the vertical direction.
The photoelectron image is accelerated and collided with the output phosphor screen after secondary electron multiplication by the MCP. The fluorescence image is observed with a high-sensitivity CCD camera and analyzed by a computer. Because the ultrafast time information is converted into two-dimensional spatial information, response speed of the reading device can be slow. The strong point is that it is possible to simultaneously acquire information other than time by positioning the spatial information that has been enlarged by means of a microscope or the wavelength information of the image in the direction perpendicular to the sweep direction.
Fig. 5.1.4. Streak camera measurement system schematic.
There are two methods of photo-multiplying sweep. One is the single sweep method which changes the applied voltage linearly at a very short switch time. The other is synchronized scan method which applies a high-voltage sine wave that is in synchronism with the repetition of the optical pulse. The former can obtain a time resolution of under 200 fs. However, the sweeps maximum repetition frequency is limited to a few hundred Hz. The latter’s time resolution is inferior to the former. However, it is possible for the sweeps maximum repetition frequency to be about a few dozen to a hundred MHz. Also, because the sweep information is superimposed on the memory, the latter can increase the signal-to-noise ratio. The time resolution of the streak tube itself is less than 500 fs. However, the actual time resolution is limited by the jitter of the whole measurement system. It is necessary to appropriately select the method according to the measuring light pulse’s repetition frequency, required time resolution or observation time.
Sampling optical oscilloscopes that combined photomultiplier tube and streak camera in order to get a high dynamic range has also been developed. With this method it is possible to distinguish whether the vibration of the measured waveform derives from the relaxation oscillation of the light or electrical signal which was difficult with conventional sampling oscilloscopes.