JPH0650847A - Method and apparatus for measuring resonator dispersion - Google Patents

Abstract

PURPOSE:To measure wavelength dispersion of a laser resonator without depending on the structure of the resonator and, besides, on a response time of a measuring circuit. CONSTITUTION:A light flux generated by a laser resonator 101 to be measured is made to enter a Michelson interferometer 102 to 105 of which a relative optical path difference 107 between two arms is set to be about an integral multiple of the resonator length of the laser resonator 101, and the intensity of an interference light generated is measured by a photodetector 108. The waveform of the intensity of the interference light in relation to a change in the relative optical path difference 107 is subjected to Fourier transform and the wavelength dispersion characteristic of the laser resonator 101 is measured.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is used for the development and adjustment of an ultrashort optical pulse laser which generates an optical pulse having a time width of picosecond or less. In particular, it relates to a technique for measuring the chromatic dispersion characteristics of such a laser resonator.

[0002]

2. Description of the Related Art Recently, a technique for generating an optical pulse having a time width of picosecond or less has been actively developed. As a result, when generating or transmitting a pulse with a short time width,
It has been clarified that the chromatic dispersion characteristics of the optical component used for generating or transmitting the optical component or the optical path which is an assembly of the optical components has a great influence on the pulse shape. For example, when a short optical pulse passes through an optical path where the chromatic dispersion characteristic changes abruptly, a waveform deformation phenomenon occurs. Further, if an optical component whose wavelength dispersion characteristic changes rapidly is used, it becomes difficult to generate an optical pulse having a short time width in the first place. In particular, in optical components inside the cavity of an ultrashort optical pulse laser, since the deformation of the waveform is accumulated many times as the light circulates in the cavity, it is necessary to control the chromatic dispersion characteristics with high accuracy. Is. Under such circumstances, it is necessary to measure the wavelength dispersion characteristic of the laser resonator with high accuracy.

As one method of measuring the wavelength dispersion characteristic of the laser resonator, a method of measuring the wavelength dispersion of each optical component in the resonator with high accuracy and estimating the wavelength dispersion of the resonator as the sum of them is considered. To be As a dispersion measuring method for this individual element, for example, in Japanese Patent Application Laid-Open No. 2-134543 (Japanese Patent Application No. 63-287566), "Dispersion measuring method and its apparatus", there is a method for measuring the measurement on one arm of an interferometer using white light. A technique is disclosed in which an element is inserted, an interference waveform generated by changing the delay time difference is recorded, and the chromatic dispersion of the device under measurement is obtained from phase information in the frequency domain obtained by Fourier transforming the recorded waveform. Regarding the waveguide type element, Japanese Patent Application Laid-Open No. 3-216530 (Japanese Patent Application No. 2-216530).
11813), "Method and apparatus for measuring waveguide dispersion",
An optical coupling optical system was installed in both arms of the white light interferometer disclosed in JP-A-2-134543 to cancel the wavelength dispersion of the optical coupling optical system for entering and exiting light into the waveguide. A measuring instrument is disclosed.

However, there are some components in the laser resonator whose wavelength dispersion characteristics largely depend on the incident direction or position of light incident on the device. For example, the optics letter (Opti
csLetter) Vol. 9, pp. 150-152, 1984, the prism pair has two isosceles prisms whose apex angles are formed so that the light entrance and exit angles are Brewster's angles. And an anomalous dispersion characteristic in which the group delay time increases with respect to wavelength is generated. At this time, the amount of dispersion generated is very much dependent on the optical path length through the glass in the prism, and hence the position of incidence of light on the prism. In such a case, it is virtually expected that the incident conditions when measuring the chromatic dispersion of an individual device and the incident conditions when the device is actually used in the laser resonator are set to be exactly the same. This is not possible, leaving a large ambiguity in estimating the chromatic dispersion of the resonator. Further, it is a problem in terms of labor and time to measure each of the parts in the shaker, which requires at least three pieces or more.

On the other hand, there is also proposed a technique for measuring the chromatic dispersion of the laser resonator itself (in this specification, the chromatic dispersion of the laser resonator itself is particularly referred to as "resonator dispersion"). Such an example is shown in FIG.

The conventional resonator dispersion measuring method shown in FIG.
Optics Letter Magazine Volume 17 514-516 1
It was used in 992 to measure the chromatic dispersion of a titanium sapphire laser resonator.

In this method, the measured laser resonator 5
Reference numeral 01 is composed of a laser medium 502, a wavelength selection device 503, an end face total reflection mirror 504, and an output coupling mirror 505,
When the laser medium 502 is excited by the continuous excitation device 506, the measured laser resonator 501 performs pulse oscillation. As the continuous excitation device 506, a continuous wave laser light source, a continuous lighting lamp or a continuous current injection source is used. The oscillation wavelength λ of the measured laser resonator 501 is controlled by the wavelength selection device 503 provided therein.

The output of the laser resonator 501 to be measured enters the photodetector 508 as an output pulse train 507 from the output coupling mirror 505 and is converted into an electric signal pulse train. This electric signal pulse train is supplied to the frequency counter 509, and the pulse repetition frequency f (λ) of the pulse train is measured.
By repeating this measurement with the wavelength selector 503 while sequentially changing the oscillation wavelength λ, the pulse repetition frequency f (λ) for each wavelength is obtained.

Here, the repetition frequency f (λ) is expressed as f (λ) = c / L (1) using the cavity optical length L of the laser cavity 501 to be measured. c is the speed of light in vacuum. The resonator group delay time τ _d is represented by an equation τ _d = (1 / c) [L−λ (dL / dλ)] (2) including the wavelength derivative of the resonator optical length. By rewriting this into an expression including the repetition frequency, τ _d = (1 / f) [1+ (λ / f) (df / dλ)] (3) is obtained. What is the dispersion characteristic of the resonator?
The third expression is a basic expression representing the principle of measuring the resonator dispersion characteristic in the conventional method, which is the change of _{d with} the wavelength λ.

[0010]

However, the above-mentioned conventional resonator dispersion measuring method has the following problems.

The first problem is that the laser resonator to be measured needs to be excited by the continuous pumping device and pulse-oscillated in this state. This requirement is not always met when considering various laser devices.

There are at least three types of methods for realizing pulse oscillation in a laser: forced mode locking, composite mode locking, and passive mode locking. In the forced mode-locking and the composite mode-locking, a modulation signal or an excitation pulse that repeats at a time interval equal to the laser cavity circulation time is externally applied. In this case, the obtained repetition frequency of the pulse oscillation always matches exactly with the repetition signal applied from the outside, and does not depend on the oscillation wavelength.
Therefore, it is not possible to measure the laser resonator adopting the forced mode locking or the composite mode locking by the conventional resonator dispersion measuring method.

The second problem is that the laser resonator to be measured must have a wavelength selection device for controlling the oscillation wavelength therein.

In general, a laser device oscillates at a fixed wavelength determined by the balance between the wavelength dependence of the gain of the laser medium and the wavelength dependence of the loss in the resonator. Since the dispersion characteristic of the resonator to be measured now is a quantity including the wavelength derivative of the repetition frequency as seen in the third equation,
It is essential to measure the repetition frequency at least at two different oscillation wavelengths. Therefore, it is necessary to forcibly change the oscillation wavelength by using the wavelength selection device in the resonator.

In a certain type of laser medium, for example, a semiconductor, it is known that the wavelength dependence of gain changes depending on operating conditions such as pumping intensity or temperature, and the oscillation wavelength can be changed by changing those conditions. May be considered. However, such a change in the operating condition of the laser medium inevitably accompanies a change in the dispersion characteristic of the laser medium, and therefore it is not allowed to change the operating condition. This is because the resonator dispersion characteristic under a constant operating condition is the measurement target here. Therefore, it is essential that the wavelength selection device is installed in the laser resonator to be measured, and, in light of the purpose of the measurement here, the change in the dispersion characteristic of the element itself due to the wavelength selection is neglected. A wavelength selection device that is as small as possible is needed.

However, in the ultrashort optical pulse laser, such a device is often not used because the wavelength selecting device in the resonator is accompanied by the expansion of the width of the generated pulse more or less. In that case, it is not convenient to temporarily install the wavelength selection device only for the purpose of measuring the resonator dispersion, and moreover, to obtain the dispersion of the resonator in a normal use state in which the wavelength selection device is not installed. In addition, it is necessary to measure the dispersion measurement of the wavelength selection device by using the dispersion measurement method for individual elements. Further, in some cases, for example, as in a monolithic mode-locked semiconductor laser, it is impossible in the first place to additionally install a wavelength selection device in the resonator after manufacturing the laser.

These first and second problems limit the object of measurement. In addition to this limitation, the conventional resonator dispersion measurement method requires that the photodetector used be sufficiently responsive to the resonator circulation time. This requirement can be easily satisfied by a commercially available PIN photodetector when the laser has a long cavity length of, for example, 1.5 m, that is, when the cavity circulation time is about 10 nanoseconds. In fact, measurements have been made on such long-cavity lasers in the documents mentioned above. However, a short cavity laser, for example, having an element length of 30
With a semiconductor laser of about 0 μm, the cavity round trip time is about 7
As short as a picosecond, there is currently no photodetector responsive to it. Therefore, such a short cavity laser cannot be measured by conventional techniques. The same high-speed response is required for the frequency counter arranged in the subsequent stage of the photodetector, which also causes great difficulty in the measurement of the short cavity laser.

As described above, in the above-described conventional resonator dispersion measuring method, the laser resonator to be measured has (1) pulse oscillation in a continuous pumping state, and (2) a wavelength selecting device with a slight change in dispersion characteristic. The extremely strong condition of controlling the oscillation wavelength by means of is imposed, and the laser resonator to which it is applicable is extremely limited. Furthermore, it is difficult to measure a short-resonance laser because the photodetector and frequency counter used must respond sufficiently to the cavity round trip time.

An object of the present invention is to solve these problems and to provide a resonator dispersion measuring method having versatility. In particular, it does not depend on the type of means for causing the measured laser resonator to perform pulse oscillation such as continuous pumping, modulation signal application or pulsed pumping, and whether or not there is a wavelength selection device in the measured laser resonator. Another object of the present invention is to provide a resonator dispersion measuring method in which the length of a measurable laser resonator to be measured is not limited by the response time of a photodetector or an electronic circuit.

[0020]

According to the resonator dispersion measuring method of the present invention, a parallel light beam is split into two light beams, and one of the two light beams is propagated to a first optical path whose optical path length is fixed. The other of the two light beams is propagated to the second optical path whose optical path length is variable, and the light propagating in the first optical path and the light propagating in the second optical path are combined and interfered with each other. Intensity is measured while changing the optical path length of the second optical path, in the resonator dispersion measurement method to obtain chromatic dispersion characteristics from the phase information in the frequency domain obtained by Fourier transforming the measured light intensity,
The parallel light beam is a light beam emitted from a laser resonator to be measured (hereinafter referred to as “measured laser resonator”), and the relative optical path length difference between the first optical path and the second optical path is this laser resonator. The optical path length of the second optical path is changed in the vicinity of an integral multiple of the resonator length of.

When the product of the divergence angle of the light beam generated by the laser resonator to be measured and the resonator length of the resonator is equal to or larger than the diameter of the light beam immediately after the resonator, that is, in the case of a diffused light beam, the resonance is generated. It is desirable to provide an optical device for enhancing the parallelism of the light flux at the output of the container.

This method can be carried out by utilizing a conventional Michelson interferometer when the resonator length of the laser resonator is short. However, when the cavity length of a dye laser or the like is long, for example, the cavity length of 1 m or more cannot be measured. In such a case, the apparatus for carrying out the method described above, that is, the Michelson interferometer is provided with an incident end on which the light beam from the laser resonator to be measured is incident, and two optical path length sweeping means are provided. A resonator dispersion measuring device is required which is set in the vicinity where the range of changing the relative optical path length difference of the optical path is an integral multiple of the resonator length of the laser resonator other than zero.

[0023]

[Function] After the parallelism of the luminous flux generated by the laser resonator to be measured is increased if necessary, the relative optical path difference between the two arms is an integer multiple of the resonator length of the laser resonator to be measured. It is incident on a Michelson interferometer set in the vicinity of, and the intensity of the generated interference light is converted into a voltage value by a photodetector and measured. At this time, each time the relative optical path length difference of the Michelson interferometer changes by a certain amount, the output voltage value of the photodetector is recorded in a time-sequential manner, and this recorded data is Fourier-transformed to obtain a frequency range in the frequency domain. The chromatic dispersion characteristic of the laser resonator to be measured is measured from the phase information. As a result, the measurable laser resonator can be measured irrespective of the type of means for causing the measured laser resonator to perform pulse oscillation and the presence or absence of the wavelength selection device in the measured laser resonator. The resonator dispersion characteristic can be measured without the length being limited by the response time of the photodetector or the electronic circuit.

This method uses a Michelson interferometer,
It is similar to the technique disclosed in Japanese Patent Laid-Open No. 2-134543 in that the wavelength dispersion of the device under measurement is obtained from the Fourier transform of the interference waveform. However, the device to be measured, in this case the laser resonator, is placed outside the Michelson interferometer, not inside, and the light incident on the Michelson interferometer is not white light but the light flux from the laser resonator to be measured. There are some major differences. Therefore, the measurement principle is also completely different from the technique disclosed in the above publication. This measurement principle will be described below.

The inventor of the present application examined what caused the problems in the above-mentioned conventional measuring method. As a result, in order to obtain the resonator optical length for each wavelength of the measured laser resonator as the primary measurement quantity, the conventional measurement method uses the phenomenon of pulse repetition frequency in pulse oscillation. It turns out that it does not occur universally.

As a method of avoiding such a problem and measuring the resonator optical length for each wavelength of the laser resonator to be measured through another phenomenon, the following method can be considered. Suwanachi,
First, a wavelength selection device, specifically, a spectroscope is installed between the laser resonator to be measured and the photodetector. By this,
No wavelength selection device is required in the laser cavity to be measured.
The output electrical signal of the photodetector is then measured by a spectrum analyzer rather than a frequency counter. This eliminates the need for the laser resonator to be measured to perform pulse oscillation. With such a configuration, even if the laser resonator to be measured oscillates continuously, or even if it does not oscillate in the first place, the resonator for the light of the wavelength selected by the spectrometer is displayed on the spectrum analyzer. A resonance peak is observed at a frequency position that is an integral multiple of the longitudinal mode frequency interval.
Since the cavity longitudinal mode frequency interval here is exactly equal to the pulse repetition frequency in the above-mentioned conventional measuring method, the dispersion characteristic of the laser cavity to be measured can be obtained by applying this to the third equation. .

However, this method also has the following difficulties.

First, in this method as well as in the conventional measuring method, it is necessary that the photodetector used can sufficiently respond to the cavity round-trip time. The same high-speed response is, of course, required also for the spectrum analyzer connected to the subsequent stage of the photodetector, and this situation is similar to the demand for the frequency counter in the conventional measurement method.

Secondly, in this case, the resonance peak observed on the spectrum analyzer at a frequency position which is an integer N times the resonator longitudinal mode frequency interval is the complex electric field amplitude at time t of the kth longitudinal mode of the resonator. Is written as A _k (t), it is proportional to Σ W (λ _k ) ∫A _k (t) A ^* _{k + N} (t) dt (4). However, Σ represents the total sum about k, and W
(λ _k ) represents the power transmittance of the spectrometer for the wavelength λ _k of the _kth mode. This peak is generally weaker than the peak appearing at the zero frequency position on the spectrum analyzer. The integral term of the fourth equation with respect to the peak appearing at the zero frequency position has the form of ∫ | A _k (t) | ² dt (5), and the absolute value is given to the integrand of the integral term of the fourth equation. The added integral ∫ | A _k (t) || A _{k + N} (t) | dt (6) has substantially the same magnitude as in the fifth equation. However, the value of the integral ∫A _k (t) A ^* _{k + N} (t) dt (7) included in the fourth equation is generally smaller than the value of the integral of the sixth equation. This is because the relative phase between the electric fields A _k (t) and A _{k + N} (t) of different longitudinal modes changes momentarily. However, when the laser resonator to be measured is performing mode-locking operation, the relative phase between the mode electric fields is fixed, so that the integral value of equation (7) becomes equal to the integral value of equation (6). A large resonance peak can be observed at, and the cavity longitudinal mode frequency interval can be easily measured. On the other hand, when the laser resonator to be measured is continuously oscillating or has not yet oscillated, since there is almost no relative phase relationship between the mode electric fields, the resonance peak is at the zero frequency position, for example. It is as small as 1 / 100,000 to 1 / 100,000 of the peak that appears, and it is practically very difficult to determine the cavity longitudinal mode frequency interval from the peak.

The inventor has found that the principle advantages of this method are:
That is, the wavelength selection device is not necessary, the property that the laser resonator to be measured does not need to perform pulse oscillation, and as a result of diligent study to eliminate the above-mentioned difficulties, the present invention using an interferometer was achieved. It was

In the interferometer, a highly accurate time axis can be generated through the relative optical path length difference between the two arms, that is, the delay time difference. The accuracy of this optical path length difference is 1μ even in a simple system.
m, several nm can be obtained by using the interferometry, which corresponds to a delay time difference accuracy of 3 to 0.02 femtoseconds. The time response of measurement is determined by this delay time accuracy, and the time response of the photodetector that receives the light emitted from the interference system is completely unrelated to the time response of measurement. This makes it possible to measure even a short resonator.

When the delay time difference of the interferometer is τ, the generated interference signal S (τ) is Σ exp (jω _k τ) ∫ A _k (t) A ^* _k (t-τ) by using the above symbols. ) dt It is proportional to (8). Where Σ represents the summation over k, and ω
_k is the angular frequency of the kth longitudinal mode of the resonator. 8th
The meaning of the equation is that the interference waveform derived from each longitudinal mode oscillating at ω _k with respect to the delay time difference is superimposed with the amplitude ∫ A _k (t) A ^* _k (t-τ) dt (9). That's what it means. The amplitude of this ninth equation is similar to the above-mentioned fifth equation because it is a correlation between the same longitudinal mode electric fields. Therefore, there is no inconvenient property as expressed by the formula (7), that is, the property is large only when the measured laser resonator is performing the mode-locking operation. Therefore, by using this interference signal for chromatic dispersion measurement, even if the laser resonator to be measured is continuously oscillating or has not yet oscillated, measurement can be performed without any difficulty. The ninth equation can be regarded as t ^N (ω _k ) ∫ | A _k (t) | ² dt (10) when the delay time difference τ is near an integer N times the resonator circulation time. Where t (ω _k ) is a complex number that represents the change that the electric field undergoes when it goes around the resonator.
Its absolute value corresponds to the change in electric field strength, and its phase corresponds to the change in phase. The integration in the latter half of the equation 10 is the optical power of the kth longitudinal mode, that is, the optical spectrum U
proportional to (ω _k ). Then, the tenth expression is changed to t ^N (ω _k ) U (ω
_k ) and using this in the equation 8, the interference signal S appearing when the delay time difference τ appears near an integer N times the resonator round-trip time
_N (τ) is expressed as S _N (τ) = Σ exp (jω _k τ) t ^N (ω _k ) U (ω _k ) ... (11) When this signal is Fourier transformed and only the component oscillating at ω _k is extracted, t ^N (ω _k ) U (ω _k ) is obtained. The optical spectrum U (ω _k ) is always a positive real number. Therefore, the phase of the Fourier transform result of the interference signal S _N (τ) as a complex number is always the phase of t (ω _k ), that is, the phase change φ that the electric field receives when it goes around the resonator.
Only (ω _k ) is reflected. That is, if the Fourier transform is represented by F, then arg (F [S _N (τ)]) = Nφ (ω) (12). From the phase change φ (ω) obtained here, the resonator group delay time τ _d is determined by the following: τ _d (ω) = dφ (ω) / dω (13) Therefore, the twelfth formula is a basic formula representing the principle of the resonator dispersion measurement according to the present invention.

[0033]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a diagram showing a basic configuration for carrying out the present invention.

The parallel light flux generated by the laser resonator 101 to be measured enters a Michelson interferometer composed of a semitransparent mirror 102, a fixed mirror 104, a correction plate 103 and a movable mirror 105. The light that has passed through the Michelson interferometer is incident on the photodetector 108, and its intensity is measured. The relative optical path length difference 107 between both arms of the Michelson interferometer is changed by moving the movable mirror 105 near a position where the relative optical path length difference 107 is equal to an integer N times the length of the laser resonator to be measured. Then, in the output signal of the photodetector 108, vibration due to the interference phenomenon of light appears. This output signal is sent to the waveform storage device 10
Memorize every 9th.

After that, the waveform of the signal recorded in the waveform storage device 109 is Fourier analyzed by the computer 110. The phase for each angular frequency of light obtained as a result of this analysis,
That is, the phase of the Fourier component is the measured laser resonator 10
A phase characteristic corresponding to an integer N rounds of 1 is given.

Next, the principle of the present invention will be described in more detail.

Now, the delay time difference of the Michelson interferometer is swept from -T to T around the point where the relative optical path difference is equal to N times the length of the laser resonator 101 to be measured (see FIG. 1). (The movable mirror 105 is moved backward in the arrangement of 2), and the time step ΔT during the sweep (when expressed by the relative optical path length difference, this ΔT may be multiplied by the light velocity c).
When the interference signal is sampled every time, the number of data points of the sampled interference signal is 2T / ΔT. When this signal data is Fourier-transformed by the computer 110, the angular frequency step Δω (= π / 2) according to the well-known sampling theorem.
T), it is decomposed into N / 2 Fourier components from ω = 0 to ω = π / ΔT. The upper end ω _NYQ of the angular frequency that appears here is the well-known Nyquist frequency in the form of the angular frequency. This upper end ω _{NYQ needs} to be larger than the angular frequency ω _L = 2πc / λ _{L of} the light corresponding to the short wavelength end λ _L of the light generated from the laser resonator 101 to be measured, that is, ω _NYQ > ω _L. is there. This prevents aliasing due to the Fourier transform of discrete data (aliasing), that is, when there is a frequency component exceeding the Nyquist frequency in the source signal, that frequency component is folded back and overlaps the portion below the Nyquist frequency. This is because. The time step Δ during sweeping this condition
When rewritten to the condition of T, ΔT <λ _L / 2c (14). Further, when converted to the optical path length difference step ΔL, the condition of ΔL <λ _L / 2 (15) is derived. It can be understood from the expression (15) that, for example, when the short wavelength end of the fluorescence from the laser resonator 101 to be measured is 1200 nm, it is necessary to perform the measurement with the step of the optical path length difference finer than at least 600 nm. For this purpose, it is essential to measure the optical path length difference of the interferometer with an accuracy of at least several tens of nm.

The following methods are conceivable for realizing highly accurate optical path length difference measurement.

The first method is to use a two-frequency He-Ne stabilized laser which is already widely used.
With this method, a resolution of 5 to 10 nm is achieved, and therefore, using this technique, the optical path length difference with the accuracy required by the present invention can be measured. However, in this length measuring method, since the semi-transparent mirror 102 in the interferometer is required to have a special polarization characteristic,
Rather, the method described below is advantageous.

In the second method, a linearly polarized monochromatic laser light source, for example, a He-Ne laser, is used as a reference light source of length, and the linearly polarized light is converted into circularly polarized light by one arm of the interferometer and the generated interference is generated. This is a method of measuring the light by separating the polarized light. With this method, two interference signals having a phase difference of 90 degrees from each other are obtained. By using these two interference signals, the measurement resolution of 1/50 or more of the wavelength of the reference light source can be easily achieved.

In addition, high resolution can be obtained even if a single interference signal is measured by a phase locked loop (PLL) without conversion into circularly polarized light. However, this method requires higher uniformity of the sweep speed of the interferometer than other methods.

The present invention can be similarly implemented by using a method other than these three methods as a method for measuring the optical path length difference of the interferometer.

FIG. 2 is a diagram showing a concrete embodiment of the present invention.

In this particular embodiment, the Michelson interferometer comprises a cube beam splitter 205, a movable mirror 20.
7 and a fixed mirror 206. In the cube beam splitter 205, the thicknesses of the glasses that pass the transmitted light and the reflected light are always equal to each other, and the correction plate 103 as shown in FIG. 1 is unnecessary. Between the Michelson interferometer and the measured laser resonator 201, a single-mode optical fiber 202 and a coupling between both ends thereof are provided as an optical device for increasing the parallelism of the light beam from the measured laser resonator 201. The lenses 203 and 204 are inserted.

As the linearly polarized monochromatic laser light source 211,
Here, a He-Ne laser is used. The reflector 212 is
The linearly polarized light beam emitted from the monochromatic laser light source 211 is reflected toward the cube beam splitter 205. 1/8
The wave plate 213 converts the linearly polarized light reflected by the cube beam splitter 205 via the reflecting mirror 212 into circularly polarized light. The reflecting mirror 214 reflects the interference light, which is obtained by combining the linearly polarized light that has passed through the fixed mirror emitted from the Michelson interferometer and the circularly polarized light from the ⅛ wavelength plate 213, in the orthogonal direction. The polarization beam splitter 215 splits the interference light reflected by the reflecting mirror 214 into polarization components. Photodetectors 216 and 217
Detects the light intensity of the interference light of each polarization component separated by the polarization beam sputter 215. The trigger generator 218, based on the two interference signals output from the photodetectors 216 and 217 and having a phase difference of 90 degrees with each other, each time the interferometer optical path length difference changes by a predetermined step length. Generate a trigger signal. Waveform storage device 219
Is synchronized with the trigger signal from the trigger generator 218,
The level value (output voltage value) of the output signal of the photodetector 210 is captured and stored.

In this structure, I in the 1.3 μm wavelength band
When measuring the wavelength dispersion of the nGaAsP semiconductor laser resonator, it is preferable to use a germanium photodetector as the photodetector 210. Further, when such a semiconductor laser is driven by a direct current, the fluorescence generated from the resonator is located at approximately 1.25 to 1.35 μm, and therefore the above-mentioned formula 15 is satisfied for this short wavelength end of 1250 nm. In order to prevent the aliasing phenomenon, the interference signal may be measured at intervals of the interferometer optical path length difference of less than 625 nm.

In this example, the above-mentioned second method is adopted as a length measuring method for achieving this step difference in optical path length with high accuracy. A monochromatic laser light source 21 is used as the length reference light source.
1. Specifically, a linearly polarized He—Ne laser that oscillates at a wavelength of 632.8 nm is used. This monochromatic laser light source 21
The laser light from No. 1 is linearly polarized in the direction of 45 degrees on the paper surface, and is reflected by the reflecting mirror 212 in parallel with the light from the laser resonator 201 to be measured via an optical device for increasing the parallelism of the light flux. It is incident on the interferometer. In this Michelson interferometer, the laser light from the monochromatic laser light source 211 is emitted from the 1/8 wavelength plate 21 in one arm of the interferometer.
3, the light is reflected by the movable mirror 207, and again passes through the 1/8 wavelength plate 213 in the opposite direction. Therefore, an effect equivalent to passing through the quarter-wave plate is produced, and linearly polarized light is converted into circularly polarized light.

The laser light having a wavelength of 632.8 nm emitted from the interferometer passes through the reflecting mirror 214 and the polarization beam splitter 21.
5 and is separated into a polarization component perpendicular to the paper surface and a component parallel to the paper surface, and the individual photodetectors 216 and 217 convert the light intensity of each component into a voltage value. The two voltage signals are input to the trigger generator 218. Trigger generator 2
In 18, a single voltage pulse is generated as a trigger signal from the two voltage signals each time the interferometer optical path length difference changes by a half of 632.8 nm, that is, 316.4 nm. With this trigger signal, the waveform storage device 219 sequentially stores the output voltage value of the photodetector 210 at the time when the voltage pulse is generated. The voltage signal value stored in the waveform storage device 219 is read by the computer 220 and subjected to Fourier transform.

FIG. 3 and FIG. 4 show examples of the measurement results.
FIG. 3 shows an interference waveform obtained at the output of the photodetector 210, and FIG. 4 shows a chromatic dispersion characteristic obtained by calculating the interference waveform. The interference waveform shown in FIG. 3 is an interference waveform output from the photodetector 210 by sweeping the movable mirror 207 for a long distance before and after the fixed mirror reference position 208 where the distance from the cube beam splitter 205 is equal to the fixed mirror 206 side. It is a record of the whole state of.

In the interference waveform shown in FIG. 3, a large interference signal S _{0 is} present near the interferometer optical path length difference (delay time difference), that is, near the position where the movable mirror 207 coincides with the fixed mirror reference position 208. (τ) appears. Also, on both sides of it,
The mutual interference signal S ₁ (τ) is separated by the orbiting time of the measured laser resonator 201 (7.46 picoseconds in this case),
S _-1 (τ) appears. Here, the mutual interference signal generated when the position of the movable mirror 207 is set back from the fixed mirror reference position 208 is S ₁ (τ). This mutual interference signal S
₁ (τ) is the measurement target here, and the delay time difference corresponding to the center is the cavity round trip time.

In the operation of the embodiment shown in FIG. 2, in order to find the position of the movable mirror where the interference signal waveform should be recorded, in the case of the short resonator as in this example, a large interference signal S is first obtained.
_It is effective to find a movable mirror position that coincides with the fixed mirror reference position 208 as a position where ₀ (τ) appears, and then retract the movable mirror position from that position to find the mutual interference signal S ₁ (τ).

In the case of a long resonator length exceeding several cm, this method is not practical because the moving range of the movable mirror moving mechanism is limited. Therefore, for example, the following method is used.
That is, the length of the resonator to be measured is calculated in advance from the thickness and refractive index data of the optical components in the resonator and the distance between them, and the movable mirror 207 and the fixed mirror 20.
The movable mirror position is tentatively set so that the difference in distance from the respective facing surfaces of the cube beam splitter 205 of No. 6 becomes equal to the calculated measured resonator length. Next, while moving the movable mirror position back and forth around that position, the photodetector 2
The output voltage value of 10 is observed and the position where the mutual interference signal S ₁ (τ) appears is found.

The overall operation of the embodiment shown in FIG. 2 will be described in more detail.

First, the movable mirror 207 is moved to the mutual interference signal S.
Advance from the observed position of ₁ (τ) to the position where the signal disappears sufficiently. Here, the memory of the waveform storage device 219 is erased, and the data write position is set to the waveform storage device 21.
Reset to the first address of No. 9. Next, when the movable mirror 207 is slowly retracted, a trigger signal is supplied to the waveform storage device 219 every time the interferometer optical path length difference changes by 316.4 nm, and the output voltage signal value of the photodetector 210 is changed to the waveform storage device 2.
It will be remembered in 19.

Here, "slowly" means the trigger generator 2
This is a sweep speed within a range in which the analog / digital conversion and writing operations of the waveform storage device 219 can follow the repetition of the trigger signal generated from 18. For example, when the analog / digital conversion and writing speed of the waveform storage device 219 is 20 kHz,
Maximum possible interferometer optical path length change speed is 20000 [/ sec]
× 316.4 [nm] = 6.1328 mm / sec,
The maximum possible moving speed of the movable mirror 207 is 3.1, which is half of this.
64 mm / sec. The half here is the movable mirror 2.
This is because the light is folded back on the surface of 07, and the amount of movement of the movable mirror 207 is twice the optical path length change.

The change range of the delay time difference required for the measurement is about twice the change amount of the group delay time of the measured laser resonator 201 in the wavelength range of the generated fluorescence. For example, in the 1.3 μm wavelength band InGaAsP semiconductor laser resonator illustrated in FIG. 2, the total change amount of the group delay time is about 0.5 picoseconds at the most in the fluorescence wavelength range of 1.25 to 1.35 μm. The required delay time difference change range is about 1 picosecond. This is also confirmed from the fact that the observed delay time range of the mutual interference signal S ₁ (τ) does not exceed 1 picosecond in FIG.

In the actual measurement, sweeping was performed over the delay time difference change range of 4.4 picoseconds with an allowance. Converting this delay time difference change range into an optical path length difference change range, 0.66 m
m, and if this range is swept with the maximum possible moving speed of the movable mirror 207, the time required for signal measurement is only about 0.1 seconds. Even if the sweep is delayed with a margin, signal measurement is completed within 1 second. A known moving mechanism was used for this sweeping. Further, the number of data points collected at this time is 4016 points, and the calculation of the Fourier transform of this data is performed by the computer 220 by 2-3.
It could be done in seconds. An example of the obtained wavelength dispersion characteristics is shown in FIG. This chromatic dispersion characteristic, that is, the measurement of the wavelength dependence of the group delay time was completed within 5 seconds in total, and extremely quick chromatic dispersion characteristic measurement was realized.

[0058]

As described above, the resonator dispersion measuring method of the present invention does not depend on the kind of means for causing the laser resonator to be measured to perform pulse oscillation, and the wavelength selecting device in the laser resonator to be measured. In addition, the measurable length of the measured laser resonator is not limited by the response time of the photodetector or the electronic circuit, and the dispersion characteristic of the laser resonator can be measured for general purposes. INDUSTRIAL APPLICABILITY The present invention can be utilized not only in the test at the time of developing an ultrashort optical pulse laser but also in the test after manufacturing, and has a great industrial effect.

[Brief description of drawings]

FIG. 1 is a diagram showing a basic configuration for carrying out the present invention.

FIG. 2 is a diagram showing a specific example.

FIG. 3 is a diagram showing a result of measuring a resonator dispersion of an InGaAsP semiconductor laser resonator, showing a result of measuring an interference waveform.

FIG. 4 is a diagram showing a result of measuring a cavity dispersion of an InGaAsP semiconductor laser cavity, showing a chromatic dispersion characteristic obtained by performing an arithmetic operation on the interference waveform of FIG. 3;

FIG. 5 is a diagram showing a conventional resonator dispersion measuring method.

[Explanation of symbols]

 101, 201, 501 Laser cavity to be measured 102 Semi-transparent mirror 103 Correction plate 104, 206 Fixed mirror 105, 207 Movable mirror 106, 208 Fixed mirror contrast position 107, 209 Relative optical path difference 108, 210, 216, 217, 508 Photodetector 109, 219 Waveform storage device 110, 220 Computer 202 Single mode optical fiber 203, 204 Coupling lens 205 Cube beam splitter 211 Monochromatic laser light source 212, 214 Reflector 213 1/8 wavelength plate 215 Polarization beam splitter 218 Trigger generation 502 Laser medium 503 Wavelength selection device 504 End face total reflection mirror 505 Output coupling mirror 506 Continuous excitation device 507 Output pulse train 509 Frequency counter

Claims (3)

[Claims] 1. A parallel light flux is split into two light fluxes, one of these two light fluxes is propagated to a first optical path whose optical path length is fixed, and the other of these two light fluxes is a second optical path whose optical path length is variable. Propagate to the optical path, combine the light propagating in the first optical path and the light propagating in the second optical path to cause interference, and change the optical intensity of the interference light to the optical path length of the second optical path. In the resonator dispersion measurement method, in which the chromatic dispersion characteristic is obtained from the phase information in the frequency domain obtained by Fourier transforming the measured light intensity while performing the measurement, the parallel light flux is emitted from the laser resonator to be measured. Is a light flux, the relative optical path length difference between the first optical path and the second optical path is an optical path length of the second optical path in the vicinity of an integer multiple of the resonator length of this laser resonator other than zero. A resonator dispersion measuring method characterized by varying. 2. A Michelson interferometer that splits a parallel light flux into two light fluxes, propagates in different optical paths from each other, and then combines and interferes with each other, and a relative optical path between the two optical paths of this Michelson interferometer. An optical path length sweeping means for sequentially changing the length difference, a measuring means for measuring the intensity of the interference light emitted from the Michelson interferometer in accordance with a change in the relative optical path length difference between the two optical paths, and In the resonator dispersion measuring device including a calculating means for obtaining a chromatic dispersion characteristic from phase information in the frequency domain obtained by Fourier transforming the light intensity waveform, the Michelson interferometer includes a laser resonator to be measured. Is provided with an incident end from which the light flux from is incident, and in the optical path length sweeping means, the range in which the relative optical path length difference between the two optical paths is changed is an integer multiple of the resonator length of the laser resonator other than zero. Near Cavity dispersion measuring apparatus, characterized in that it is set to. 3. The resonator dispersion measuring apparatus according to claim 2, wherein the incident end is provided with optical means for increasing the parallelism of the light beam emitted from the laser resonator to be measured.