JPH1187828A - Optical pulse source of stabilized repetitive frequency - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical pulse source of a stabilized repetitive frequency for being effective to the optical pulse source provided with any high repetitive frequency, and minimizing the increase of noise by an electronic circuit. SOLUTION: A part of an optical pulse string from a laser resonator 100 is branched by a branching mirror 102 and bisected further by the branching mirror 103, and one of pulse strings is passed through a waveform shaping device 105, propagated through an optical delay line 1.06 for supplying delay equal to the integral multiple of a pulse repetition cycle, and then, made incident to a cross-correlation device 107 along with the other pulse string bisected by the branching mirror 103. The cross-correlation device converts nonlinear signals generated by making the two pulse strings be incident on a nonlinear crystal 118 to electric signals and outputs them as cross-correlation signals Gc . The difference of a cross-correlation signal voltage and a reference voltage 108 is inputted to an integration circuit 109, a resonator length adjustment mechanism 101 is driven by a driving circuit 110 based on the output of the integration circuit 109, and the resonator length of the laser resonator is adjusted.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technique for stabilizing the repetition frequency of an optical pulse source that generates an optical pulse having an ultra-short duration in the picosecond to femtosecond range. In particular, the present invention provides an optical pulse source which can be realized at low cost and has a stable repetition frequency which can be applied to an optical pulse source having a high repetition frequency.

[0002]

2. Description of the Related Art In recent years, the use of ultra-high-speed optical signals in the picosecond to femtosecond region has been prosperous in many industrial and academic fields such as optical communication and optical information processing. The demand for generated equipment has increased. The most basic form of such an ultrafast optical signal is
It is an optical pulse having a very short time width, and it is known that once such an extremely short optical pulse is obtained, it can be converted into another signal waveform.

At present, among the methods for generating ultrashort light pulses, the light pulse with the shortest time width can be generated stably at the highest repetition frequency due to the optical nonlinear effect of the laser light source itself. This is the case of a pulse generation operation, and this kind of pulse generation operation is generally called passive mode locking or self mode locking. As a method opposed to this, there is a pulse generation operation performed by applying a modulation signal to the laser light source from the outside, and active mode locking,
Or it is called forced mode synchronization.

In any of the pulse generation operations, the repetition frequency of the light pulse is an integral multiple of the reciprocal of the round-trip time of the laser resonator. In the latter generation operation of the active mode locking method, an optical pulse is grown by applying a modulation signal having a frequency that is 1 / integer or an integral multiple of the repetition frequency determined by the resonator length. Recall that the laser cavity length inevitably changes due to drift due to temperature change or fluctuation due to vibration. At this time, in the latter active mode locking method, a discrepancy occurs between the repetition frequency determined by the resonator length and the frequency of the modulation signal,
The pulse operation becomes unstable or the time width of the generated optical pulse is widened. However, the repetition frequency of the optical pulse is determined by giving priority to the frequency of the modulation signal. On the other hand, in the former passive mode-locking method, even if the laser cavity length changes, the pulse operation is stable and the time width of the light pulse remains unchanged, but the repetition frequency of the light pulse changes. .

When applying an ultrashort light pulse generated from an optical pulse source, it is often undesirable to fluctuate the repetition frequency of the light pulse. For example, when constructing a measurement system using an optical pulse, the obtained time resolution depends on the synchronization accuracy of the pulse. Therefore, if there is a jitter due to the fluctuation of the repetition frequency, the time resolution will be impaired. In such a case, the short time width of the optical pulse generated by the passive mode locking method becomes meaningless. Therefore, efforts have conventionally been made to stabilize the repetition frequency of the generated optical pulse by controlling the cavity length of the passive mode-locked laser.

FIG. 7 shows a configuration of such a conventional example.
A method is employed in which a stable reference oscillator is used and the repetition frequency of an optical pulse generated in the oscillation output is synchronized. Such a method is described in the IEEE Journal of Quantum Elec
tronics magazine, volume 28 (1992), 289-296
Page, or Optics Letters, Vol. 19 (1994)
-It is disclosed on pages 481-483.

[0007] In the example of FIG.
Is provided with a displacement mirror 522 therein so that the resonator length can be controlled by the displacement mirror 522. A part of the optical pulse train generated from the laser resonator 500 is split by the split mirror 502 and enters the high-speed photodetector 530 via the reflecting mirror 504. Assuming that the repetition frequency of the optical pulse train is f, the electric output of the high-speed photodetector 530 is
A sinusoidal beat signal having a frequency that is an integral multiple of f is superposed. Of this output, a sine wave component of m times m (m is an integer other than zero) is amplified by the high frequency amplifier 531 and used for stabilizing the repetition frequency of the optical pulse.

The sine wave component amplified by the high frequency amplifier 531 is output to the high frequency mixer 5 together with the output of the reference oscillator 532.
33. Here, the oscillation frequency f _ref of the reference oscillator 532 is set near mf. High frequency mixer 5
33 outputs the sum and difference frequencies of the two applied inputs. Of these, the frequency mf-f _ref which is the difference component
Is extracted by the low-pass filter 534 and supplied to the drive circuit 510 of the displacement mirror 522 via the integration circuit 509.

A method of stabilizing the oscillation frequency of the oscillator at _fref / m by feeding back the difference frequency mf- _fref to the frequency control terminal as an error signal with respect to the oscillator having the free-running frequency f is generally known. , Phase locked loop (PLL). Thus, in this conventional optical pulse source having a stabilized repetition frequency, the repetition frequency of the optical pulse train is changed to the oscillation frequency f of the reference oscillator 532.
_It is drawn to 1 / m of _ref , and stabilization of the repetition frequency is realized.

[0010]

However, the above-mentioned conventional optical pulse source in which the repetition frequency is stabilized has the following problems.

In the above-described conventional stabilizing method, it is clear that better stabilization can be expected as the multiplication number m of the sine wave component used for stabilization is larger. This is because the change of the error signal mf-f _ref to a change in the repetition period f is proportional to the multiplication factor m. However, in the conventional stabilization method, the high-speed photodetector 530, the high-frequency amplifier 531
Due to the band limitation of the reference oscillator 532 and the high-frequency mixer 533, the number of multiplications m that can be set is limited.

On the other hand, considering the harmonics of the repetition frequency f included in the original optical pulse train, there are harmonics of the order of the reciprocal of the duty ratio of the pulse train, that is, the ratio of the pulse width to the repetition period. For example, a pulse train having a repetition frequency f of 200 MHz and a width of 50 fs (femtosecond) includes a harmonic of 10,000 or more. On the other hand, in the above-mentioned conventional stabilizing method, the reference oscillator 5
Even if the frequency f _ref of 32 is set to 10 GHz,
The multiplication number m is only 50 at most, and the short pulse property of the original mode-locked laser light source is not fully utilized.

Further, for a laser light source having a high repetition frequency f, for example, a mode-locked semiconductor laser with f being 100 GHz, it is difficult to set the multiplication factor m to even 1. Recall that m had to be a non-zero integer above, in which case the conventional method would not be able to achieve a stabilizing operation in the first place.

In general, as the repetition frequency f or the multiplication number m increases, the electronic circuits used, that is, the high-speed photodetector 530, the high-frequency amplifier 531, the reference oscillator 532, and the high-frequency mixer 533 become more expensive. In particular, the price of the low-noise reference oscillator 532 in the microwave band easily exceeds the price of the laser light source itself, resulting in impaired economics of the entire repetition frequency stabilized optical pulse source.

In the conventional stabilizing method, in addition to the above-mentioned band limitation and price limit of the electronic circuit, even if the stabilizing operation is achieved, the stability of the repetition frequency is, in principle, a reference. There is a fundamental constraint that the stability of the oscillation frequency of the oscillator cannot be exceeded. This limitation has not been considered as a problem with the unstable ultrashort light pulse light source which is already becoming a thing of the past, but has emerged as a significant problem with the progress of the ultrashort light pulse light source.

As described above, when an optical pulse train is received by the high-speed photodetector 530, its electric output is a superposition of sinusoidal beat signals having a frequency that is an integral multiple of the repetition frequency f. Here, if there is no fluctuation in the repetition frequency, each sine wave component has a complete line spectrum on the frequency axis. Conversely, if you observe the beat signal on the frequency axis, the phase noise power that appears around the line spectrum
Fluctuations can be detected with high sensitivity at the repetition frequency,
This measurement method is called a phase noise measurement method.

FIG. 8 shows a multiplication number m =
For the stabilized Cr-doped YAG (yttrium aluminum garnet) laser as No. 5, using the above-described phase noise measurement method, the oscillation of the repetition frequency was
The results measured before and after stabilization are shown. In FIG. 8, a broken line indicates a phase noise power spectrum of a Cr-doped YAG laser in a free-running state without performing stabilization. On the other hand, the solid line is
4 shows a phase noise power spectrum of the same laser after stabilization. When these are compared, it can be seen that in the frequency region of 700 Hz or less, the phase noise is reduced by the stabilization, and the fluctuation of the repetition frequency is suppressed by the conventional method.

However, above 700 Hz, on the contrary, the phase noise increases due to the stabilizing operation. Furthermore, even in the frequency range of 700 Hz or less, 100 Hz,
150Hz, 250Hz, 300Hz and 600Hz
, The reduction of the phase noise is remarkably inferior in the harmonics of the power supply frequency (in this case, 50 Hz). These measurement results show that the phase noise characteristic of the reference oscillator 532 used is
This indicates that the light pulse train has been transferred by the stabilizing operation. In the present ultrashort light pulse light source, the phase noise is extremely small in the self-running frequency range of several hundred Hz or more, and is equal to or superior to the phase noise of the electronic oscillator. This is due to the difference between the energy of the electromagnetic wave generated by the electronic oscillator and the optical oscillator (laser light source) as a quantum. In an ideal state, this energy is high and the optical oscillator is separated from the thermal noise region. In principle, noise can be reduced. Therefore, the conventional stabilization method, in which the noise of the electric oscillator is transferred to the laser light source as the optical oscillator, can be regarded as impairing the possibility originally given to the laser light source by the laws of physics. .

As described above, the conventional stabilized optical pulse source having a repetition frequency that pulls the pulse repetition frequency of the mode-locked laser light source to the frequency of the reference oscillator using the phase-locked loop has the following advantages. The frequency band in which the short pulse property of the mode-locked laser light source is not utilized due to the band limitation, (2) the economical efficiency is impaired by the price of the high-frequency electronic circuit, and (3) the low noise property of the laser light source is originally superior. Thus, there is a problem to be solved that results in an increase in noise.

An object of the present invention is to solve these difficult problems in the prior art, and to be effective and inexpensive for any optical pulse source having a high repetition frequency.
Another object of the present invention is to provide an optical pulse source having a stabilized repetition frequency with a minimum increase in noise due to an electronic circuit.

It is a further object of the present invention that there is no limitation due to the bandwidth of the high frequency electronic circuit, so that both the short pulse and high repetition properties of the mode-locked laser source favor the stabilization and the applicable repetition frequency. Has no upper limit,
It is still another object of the present invention to provide an optical pulse source having a stabilized repetition frequency, which has an advantage that noise is less introduced by an electronic circuit.

[0022]

In order to achieve the above object, an optical pulse source having a stabilized repetition frequency according to the present invention comprises a pulse train generated from a laser resonator, divided into two by a branching means, and one of the pulse trains. After giving a delay equal to an integer multiple of the pulse repetition period other than zero, the light is incident and focused together with the other pulse train in a medium (for example, a nonlinear crystal) exhibiting an optical nonlinear effect, and a nonlinear signal generated in the medium is converted into a nonlinear signal. It is characterized in that it is converted into an electric signal, and the cavity length of the laser resonator is controlled based on the electric signal.

As one form of this control, there is a DC method for adjusting the resonator length so that the electric signal has a constant value.

As another mode of control, a periodically changing delay is given to one of the divided pulse trains, and the length of the resonator is adjusted so that the electric signal does not change with the change of the delay. There are interchangeable ways to do it.

The delay equal to the integral multiple can be provided by an optical fiber. At this time, it is preferable to provide, at the input end of the optical fiber, an optical system for shaping the optical fiber into a pulse waveform having a small pulse width due to propagation in the optical fiber.

More specifically, according to the first aspect of the present invention, a laser resonator for generating an optical pulse train is provided with a mechanism for adjusting the optical length of the resonator, and the optical length is adjusted by adjusting the optical length. In an optical pulse source for stabilizing a repetition frequency, a branching unit for branching an optical pulse train generated from the laser resonator, and one of the optical pulse trains branched by the branching unit is provided with a delay of a non-zero integer multiple of a pulse repetition period. Delay means for providing, and optical means for incidently focusing both light pulse trains of the other light pulse train still branched by the branching means and the light pulse train delayed by the delay means into a medium having an optical nonlinear effect, Converting means for converting a non-linear signal generated in the medium having the non-linear effect into an electric signal; and an optical length of the laser resonator based on the electric signal. Characterized by comprising a drive control means for driving the adjustment to mechanism.

Here, the drive control means may drive a mechanism for adjusting the optical length of the laser resonator so that the electric signal has a constant value.

Further, the conversion means is a light detection means,
The drive control unit includes a comparison unit that compares the electric signal output from the light detection unit with a predetermined reference voltage, and the driving unit controls the electric signal based on an output signal of the comparison unit so that the electric signal matches the reference voltage. Driving means for driving a mechanism for adjusting the optical length of the laser resonator.

Further, the medium having the nonlinear effect is a nonlinear crystal or a material exhibiting a two-photon absorption effect exhibiting a second harmonic generation effect, and the converting means is a light detecting means, or the medium having the nonlinear effect is exhibiting the nonlinear effect. The medium and the conversion means may be shared by one semiconductor photodetector exhibiting a two-photon absorption effect.

Further, the delay means may be an optical fiber.

Further, a waveform shaping means for shaping a pulse waveform may be provided at an incident end of the optical fiber.

Further, a periodic delay means for giving a delay that changes periodically before giving a delay of a non-zero integral multiple of the pulse repetition period to a pulse train giving a non-zero integral multiple of the pulse repetition period. And signal extraction means for extracting only a component having the same phase as the period of the periodically changing delay in the electric signal, and the drive control means controls the resonance of the resonator based on the component having the same phase. The mechanism for adjusting the optical length can be driven.

Further, a periodic delay means for giving a periodically changing delay to the other pulse train which does not give a delay that is not an integral multiple of non-zero of the pulse repetition period; Signal extraction means for extracting only a component having the same phase as the cycle of the delay to be performed, and driving the mechanism for adjusting the optical length of the resonator by the drive control means based on the component having the same phase. it can.

Further, the periodic delay means is constituted by an oscillator and a displacement mirror connected to the oscillator, the conversion means is a light detection means, and the period of the electric signal output from the light detection means is equal to the period. The signal extracting means for extracting only a component having the same phase as the periodically changing delay period may be a phase detection amplifier connected to the oscillator.

Further, the delay means may be an optical fiber.

Further, the optical fiber may further comprise a waveform shaping means for shaping a pulse waveform at the incident end.

Further, the medium having the nonlinear effect is a nonlinear crystal or a material exhibiting a two-photon absorption effect exhibiting a second-harmonic generation effect, and the converting means is a light detecting means, or the medium having the nonlinear effect is not affected. The medium and the conversion means may be shared by one semiconductor photodetector exhibiting a two-photon absorption effect.

According to a thirteenth aspect of the present invention, there is provided a laser resonator for generating an optical pulse train, a resonator length adjusting mechanism for adjusting an optical length of the laser resonator, and a branch for an optical pulse train generated from the laser resonator. A first splitting mirror, a second splitting mirror for further splitting one optical pulse train split by the first splitting mirror, and a pulse repetition to one optical pulse train split by the second splitting mirror. An optical delay line that provides a delay that is an integral multiple of a period that is not zero, a waveform shaper that is connected to an input end of the optical delay line and shapes a pulse waveform of an input optical pulse train, and is branched by the second branch mirror. Both optical pulse trains of the other optical pulse train as it is and the optical pulse train delayed by the optical delay line are incident and focused together in a medium having an optical nonlinear effect, and a nonlinear signal generated in the medium having the nonlinear effect is generated. Electricity A cross-correlator that converts the signal into a signal, an integration circuit that compares the electric signal output from the cross-correlator with a reference voltage, and an electric signal that matches the reference voltage based on an output signal of the integration circuit. And a driving circuit for driving the resonator length adjusting mechanism.

Here, the cross-correlator applies both the optical pulse train of the other optical pulse train that has been split by the second splitting mirror and the optical pulse train delayed by the optical delay line to an optical nonlinear effect. A reflecting mirror, a right-angle reflecting prism, and a converging lens as optical means for incident focusing on a medium having the nonlinear crystal, a nonlinear crystal exhibiting a second harmonic generation effect as a medium having the nonlinear effect, and a nonlinear crystal generated by the nonlinear crystal. A diaphragm, a condenser lens, for converting signals into electrical signals,
And a photodetector.

Further, the cross-correlator has an optical non-linear effect for both the optical pulse train of the other optical pulse train that has been split by the second splitting mirror and the optical pulse train delayed by the optical delay line. A two-photon absorption device that also functions as a reflecting mirror, a right-angle reflecting prism, and a focusing lens, which are optical means for incident focusing on a medium, and a medium having the nonlinear effect and a means for converting a nonlinear signal generated in the medium into an electric signal. A semiconductor photodetector exhibiting an effect.

(Function) In the present invention, the pulse train generated from the laser resonator is divided into two, and one pulse train is given a delay Td equal to M (M is an integer other than zero) times the pulse repetition period 1 / f. After that, together with the other pulse train, the light is focused into a medium exhibiting the optical nonlinear effect, and the nonlinear signal generated in the medium is converted into an electric signal.

In general, in a medium exhibiting an optical nonlinear effect,
A signal called a cross-correlation signal is obtained by causing two light pulses to interact with each other and measuring the magnitude of the generated nonlinear signal. The order and type of the optical nonlinear effect used here, and as a result, there are various modes of the nonlinear signal. For example, as the second-order optical nonlinear effect,
The second harmonic generation effect or the two-photon absorption effect is often used.

In the former double-harmonic generation effect, half of the wavelength of the two incident lights, that is, light having a wavelength of λ / 2 with respect to the wavelength λ (double-harmonic light) is generated. It is converted by a photodetector having sensitivity to the wavelength to obtain an electric signal proportional to the power of the second harmonic light. In the latter two-photon absorption effect, the transmittance of the medium decreases with the two-photon transition that occurs when one photon is obtained from each of the two incident lights, so that the incident light is sensitive to the wavelength of the incident light. It is converted by a detector to obtain an electrical signal proportional to the change in transmittance. Alternatively, in the case of a semiconductor material, an electric signal proportional to the two-photon transition probability can be directly obtained by attaching an electrode and collecting carriers generated by the two-photon transition with the electrode.

As the third-order optical nonlinear effect, an optical Kerr effect or a coupling tone generating effect is used. The incident light is converted by an appropriate photodetector to obtain an electric signal proportional to the magnitude of each effect. .

Further, a non-resonant nonlinear effect without a real transition of one photon is usually used. In this case, the optical nonlinear medium or element can be regarded as showing an instantaneous response in practical use. Thus, for example, as a table-type cross-correlation signal G _c in the case of using the second-order optical nonlinear effect,

[0046]

(Equation 1)

Is obtained. Here, I ₁ (t) represents the intensity waveform of the pulse train to which a delay is applied, and I ₀ (t) represents the intensity waveform of the other pulse train. Integrand I ₁ (t−T
_d ) I ₀ (t) is the intensity waveform of the second harmonic generated by the nonlinear crystal, and the integral represents the effect of the photoelectric conversion by the photodetector having a sufficiently slow response time compared to the light pulse.

Here, the delay _Td is _equal to the pulse repetition period 1
Using equal to M times / f,

[0049]

(Equation 2)

The following holds. Here, Δf represents the variation of the repetition frequency, and the function ymodx represents the remainder of y modulo x. This equation (2) is substituted into the above equation (1) to obtain an equation representing the Δf dependency of the cross-correlation signal G _c.

[0051]

(Equation 3)

Is obtained.

The above equation (3) indicates that in a signal obtained by optical sampling measurement of the pulse I ₁ (t) by the pulse I ₀ (t), the delay τ between these two pulses is exactly τ = M This corresponds to the expression when it is equal to (1 / f) (Δf / f). Such an interfunction signal takes its maximum value when the delay τ is zero, and the absolute value of the delay τ is the pulse I
It is not zero only within the range of the pulse width tp of ₁ (t). That is, the range of the relative variation Δf / f of the repetition frequency at which the change of G _c is seen is:

[0054]

(Equation 4)

Is expressed as follows. Here, in parentheses on the right side,
This is nothing but the ratio of the pulse width to the repetition period, that is, the duty ratio. As mentioned above, the repetition frequency 200
In the case of a femtosecond light source having a frequency of 50 MHz and a width of 50 fs, the duty ratio is a small value of about 10 ⁻⁴ ,
If a long optical fiber delay line is used, the repetition period M = 1
A delay _{Td of} 00 times can be easily provided.

[0056] This results in the full range of the cross correlation signal G _c corresponds to the variation of the repetition frequency of 1 ppm, further 1 as the signal-to-noise ratio of the cross-correlation signal G _c
Since 000 can be expected sufficiently, a change in the repetition frequency of 1 ppb can be easily detected. In addition, since the duty ratio decreases as the pulse width decreases, the detection sensitivity in this case increases as the pulse width decreases. That is, in this method using a cross-correlation signal G _c, it can be said that the short pulses of the mode-locked laser light source is fully utilized.

As another example, a repetition frequency of 100 GH
Considering a mode-locked semiconductor laser light source having a z and a width of 1 ps, although the duty ratio is about 10 ^-1 , the repetition frequency is high, and by using an optical fiber delay line having a length of 2 km, the repetition period M = 10 ⁵ It can give a delay T _d ranging doubled, in the same manner as described above, the cross-correlation signal G _c, full range 1 ppm, enabling the detection of variations in the repetition frequency of the sensitivity 1 ppb. Even if a delay line of a fixed length is used, if the repetition frequency of the laser light source increases, the multiple M with respect to the repetition period naturally increases in proportion, so the method takes advantage of the high repeatability of the mode-locked laser light source. It is.

A method for obtaining an error signal for controlling the cavity length of the laser cavity using the above cross-correlation signal G _c includes the following.
There are two methods, a DC method and an AC method.

In the DC method, the difference between the cross-correlation signal G _c and the reference value is used as an error signal so that the cross-correlation signal G _c becomes equal to the reference value. Here, G for the variation Δf of the repetition frequency
_c has the largest rate of change, that is, G _{c at the} inflection point
Is the optimal reference value. In practice, it may be selected outline, a half value of the maximum value of G _c as a reference value.

As is apparent from the above equation (3), G _c is proportional to the pulse intensity. Therefore, there is intensity noise in the laser light source, the intensity of the generated pulses fluctuates, so that the change despite the cross-correlation signal G _c variation Δf is not the repetition frequency. At this time, if the length of the resonator is controlled by an error signal obtained by a DC method, the fluctuation of the pulse intensity causes a change in the repetition frequency.

The drawbacks of such a DC method can be avoided by using the following AC method. The AC manner, periodically gives a varying delay one-half pulse train, the cross-correlation signal G _c to adjust the cavity length so as not to change is accompanied with a change of the delay. To do this, synchronous detection G _c against periodic variation of the delay may be the detection output and the error signal. In this case, that the rate of change of G _c to variations Δf of the repetition frequency is zero, i.e.,
In the normal case where the pulse waveform I ₁ (t) has a simple bell shape, the maximum value of G _c is the stabilizing operating point. Even if the intensity of the pulse generated from the laser light source fluctuates, this stabilizing operating point does not move as long as the pulse waveform itself is kept unchanged. Therefore, in this alternating method, the stability of the repetition frequency achieved is not affected by the intensity fluctuation of the pulse.

As already mentioned, the delay T _d equal to the integer M times can be provided by an optical fiber.
At this time, chromatic dispersion (group velocity dispersion D) is applied to the optical fiber delay line.
If there are _2), the pulse width t _p during the incident, the width t _'p after propagation,

[0063]

(Equation 5)

[0064] As described above, since the feedback sensitivity is inversely proportional to the width t ′ _p after propagation of the delay path, the width t _p = 2 (D ₂ ln2) that minimizes the width t ′ _p
It is preferable to provide an optical system for adjusting the pulse width at the incident end of the optical fiber delay line so that a ^1/2 pulse is incident.

For example, if two kinds of optical fibers having different signs of the group velocity dispersion D ₂ are combined, the group velocity dispersion D ₂ of the optical fiber delay line can be made zero with respect to the center wavelength of the pulse. . However, even in this case, more of the order of the third order wavelength dispersion D ₃ remains is customary. This is because the third-order chromatic dispersion of an optical fiber usually takes a positive value. Although an optical fiber having a third-order chromatic dispersion of zero or a negative value can be designed, it is very difficult to manufacture the optical fiber because high-precision dimensional control is required. is there. Width t _'p after propagation in cubic presence of chromatic dispersion D ₃ is 2 asymptotically input pulse width t _p
It is inversely proportional to the power. Therefore, in this case, it is preferable to provide an optical system for increasing the pulse width at the incident end of the optical fiber delay line until the spread due to the third-order chromatic dispersion becomes inconspicuous.

[0066]

Embodiments of the present invention will be described below in detail with reference to the drawings.

(First Embodiment) FIG. 1 shows a configuration of an optical pulse source having a stabilized repetition frequency according to a first embodiment of the present invention. As an example of a method for obtaining an error signal for controlling the resonator length, an example of a configuration employing the above-described DC method will be described.

In the configuration shown in FIG.
Has a resonator length adjusting mechanism 101 and is configured to be able to control the resonator length. For example, in a laser resonator having an independent end mirror, an end mirror is attached to one end of a piezoelectric actuator as a resonator length adjusting mechanism 101, and the position of the end mirror is changed by a voltage applied to the press-fitting actuator. Use the device.

As this displacement mirror device, for example, the displacement mirror device described in Japanese Patent Application No. 8-232320 proposed by the present inventor can be used. The proposed displacement mirror device is
A weight is attached to the other end of the piezoelectric actuator having a reflecting mirror attached to one end, and this piezoelectric actuator is fixed by a support frame at one point (fixed point) in the longitudinal direction, and the mass M of the reflecting mirror and the distance from the fixed point to the reflecting mirror are fixed. The product M'L 'of the product ML of the distance L, the mass M' of the weight and the distance L 'from the fixed point to the weight
As a result, the reaction of the force applied to the reflector and the reaction of the force applied to the weight cancel each other on the fixed point, and as a result, no force is applied to the fixed point or the mount holding it, There is an advantage that a stable resonator length change can be realized without causing vibration and high-speed control is possible.

In the case of an optical fiber laser resonator formed by directly depositing an end mirror on the end face of an optical fiber, the resonator length adjusting mechanism 101 is used to increase or decrease the temperature of part or all of the fiber resonator. Alternatively, a configuration in which a part of the fiber is expanded and contracted by a piezoelectric actuator can be used.

In the case of a monolithic semiconductor laser resonator, the temperature is increased or decreased in the same manner as the fiber resonator.
Alternatively, a configuration in which a phase adjustment region including an electrode is formed in a resonator and the refractive index of the phase adjustment region is changed by a voltage applied to the electrode can be used as the resonator length adjustment mechanism 101.

A part of the optical pulse train generated from the laser resonator 100 is split by the split mirror 102 and used for stabilizing the repetition frequency. This branched pulse train is
The pulse train is further divided into two by the branch mirror 103 at the next stage, and one of the pulse trains enters the waveform shaper 105 via the reflection mirror 104. The waveform shaper 105 adjusts the pulse width according to the wavelength dispersion characteristics of the optical delay line 106 so that the pulse width of the subsequent optical delay line 106 at the time of emission is reduced. The pulse train output from the waveform shaper 105 propagates through an optical delay line 106 that gives a delay equal to M (M is an integer other than zero) times the pulse repetition period, and is then split into two by the split mirror 103. At the same time, the light enters the cross-correlator 107. The cross-correlator 107 performs the above-described cross-correlation signal G on the two incident pulse trains.
Output _c .

Such a cross-correlator 107 is, for example,
It comprises reflecting mirrors 114 and 115, a right-angle reflecting prism 116, a converging lens 117, a non-linear crystal 118, a stop 119, a condenser lens 120, and a photodetector 121. Here, the reflecting mirrors 114 and 115, the right-angle reflecting prism 116
Is installed to narrow the distance between the two pulse train beams incident on the cross-correlator and to impinge on the focusing lens 117. The two pulse trains passing through the focusing lens 117 intersect in the nonlinear crystal 118 located at the focal point of the focusing lens 117, and at the same time, each beam is narrowed down. Thus, the two pulse trains form the nonlinear crystal 11
8 interact in a state of high light intensity. In the present example, a second-harmonic generation effect in the nonlinear crystal 118 is used as the non-linear effect. In this case, the generated second-harmonic light is emitted in a direction sandwiched by two incident lights.
The stop 119 is arranged so as to have an opening in the direction of emission of the second harmonic light. As a result, only the second harmonic light generated by the interaction of the two pulse trains passes through the stop 119 and enters the condenser lens 120. Can be reached. The second harmonic light collected by the condenser lens 120 is applied to the photodetector 12.
It is photoelectrically converted by 1, an electric signal proportional to the magnitude of the cross correlation signal G _c (cross-correlation signal voltage) is obtained.

The cross-correlation signal voltage output from cross-correlator 107 is compared with reference voltage 108, and this reference voltage 1
08 is input to the integrating circuit 109. The optimum value of the reference voltage 108 is a value at the inflection point of the cross-correlation signal voltage, but may be set to about half of the maximum value of the cross-correlation signal voltage in practical use. The output of the integration circuit 109 is supplied to a drive circuit 110 that drives the above-described resonator length adjustment mechanism 101, and the resonator length adjustment mechanism 101 adjusts the resonator length of the laser resonator 100.

(Modification of First Embodiment) FIG. 2 shows a configuration of an optical pulse source in which a repetition frequency is stabilized, which is one of modifications of the first embodiment of the present invention. An example of a configuration using a cross-correlator 107 applying two-photon transition in a detector is shown.

Such a cross-correlator 107 includes, for example,
It comprises reflecting mirrors 114 and 115, a right-angle reflecting prism 116, a focusing lens 117, and a photodetector 130.
Here, the reflecting mirrors 114 and 115, the right-angle reflecting prism 11
Numeral 6 is provided for narrowing the interval between the beams of the two pulse trains that have entered the cross-correlator 107 and for allowing the beams to enter the focusing lens 117. The two pulse trains that have passed through the focusing lens 117 are focused on the photodetector 130 located at the focal point of the focusing lens 117.

[0077] In the present embodiment, the optical detector 130 is a semiconductor photodetector, the wavelength lambda _g corresponding to the band gap energy, with respect to the wavelength of the two pulse train incident lambda,

[0078]

Satisfies λ _g <λ <2λ _g (6) Due to this condition, the photodetector 130 has no sensitivity to an incident pulse train in the linear region.
However, in the non-linear region where two photons are involved, carriers are generated and a photocurrent proportional to G ₀ + G ₁ + G _c is generated. Here, G _c is the cross-correlation signal described above. G ₀ and G ₁ represent non-linear signals generated by the respective pulse trains alone, and are signals that do not depend on a relative delay between two pulse trains or a variation in repetition frequency.

The cross-correlation signal voltage output from the photodetector 130 of the cross-correlator 107 is compared with a reference voltage 108, and the difference from the reference voltage 108 is input to an integration circuit 109. The optimum value of the reference voltage 108 is a value obtained by adding the value at the inflection point of G _c to the constant part G ₀ + G ₁ .
In practice, no problem set about half of the maximum value of G _c to a value obtained by adding the G ₀ + G _1.

The other configuration and operation conform to the basic configuration of the above-described first embodiment shown in FIG. 1, and the description thereof will be omitted.

(Second Embodiment) FIG. 3 shows a configuration of an optical pulse source in which a repetition frequency is stabilized, according to a second embodiment of the present invention. As a method of obtaining an error signal for controlling the length, a configuration employing the above-described AC method will be described.

In the configuration shown in FIG.
0, the resonator length adjusting mechanism 101 is the same as the example in FIG.
A part of the optical pulse train generated from the laser resonator 100 is split by the split mirror 102 and used for stabilizing the repetition frequency. The branched pulse train is further divided into two by the next stage branch mirror 103. 2 by the split mirror 103
After being divided, the optical delay line 10
The displacement unit 111 is mounted on the reflecting mirror 104 that reflects the pulse train toward 6. The displacer 111 is driven by an oscillator 112 and oscillates the reflecting mirror 104 back and forth, thereby giving a periodically changing delay to the pulse train.

Alternatively, as shown in FIG.
3, the displacement unit 111 is attached to the split mirror 103 that reflects the other pulse train.
The same effect can be obtained by vibrating.

In FIGS. 3 and 4, the cross-correlation signal voltage output from cross-correlator 107 is
Is input to The output of the oscillator 112 is connected to the phase detection amplifier 113 as a reference input, and the output of the oscillator 112 of the cross-correlation signal voltage, that is, the amplitude of the component that changes in phase with the above-mentioned periodically changing delay, It is obtained as an output from the phase detection amplifier 113. The output of the phase detection amplifier 113 is input to the integration circuit 109,
The resonator length adjustment mechanism 101 is driven through the drive circuit 110.

Other structures and operations are similar to those of the above-described first embodiment shown in FIG.

As the cross-correlator 107 shown in FIGS. 3 and 4, the cross-correlator using the effect of generating a second harmonic in the nonlinear crystal used in FIG. 1 or the semiconductor light used in FIG. Any cross-correlator that uses two-photon transitions in the detector can be used as well. In the latter case, as described above, the cross-correlation signal voltage from the cross-correlator 107 is:
Although a constant part that does not depend on the relative delay between the two pulse trains is included, this constant part does not change with the above-described periodically changing delay, and therefore, does not change the output of the phase detection amplifier 113. Absent.

[0087]

Next, more specific embodiments of the present invention will be described.

(First Embodiment) FIG. 5 shows a configuration of an embodiment corresponding to the configuration of the first embodiment of the present invention described with reference to FIG.

In the configuration shown in FIG. 5, a Cr-doped YAG laser that generates a pulse train having a repetition frequency of 200 MHz and a width of 60 fs at a wavelength of 1.55 μm is used as the laser resonator 1.
00. In this case, a so-called displacement mirror 122 is used as the resonator length adjusting mechanism, in which a total reflection end mirror is attached to one end of the piezoelectric actuator, and the position of the end mirror is changed by a voltage applied to the piezoelectric actuator.

The optical pulse train with an average output of 100 mW generated from the laser resonator 100 is split into two by a split mirror 102, and one of the 50 mW optical pulse trains is used for stabilizing the repetition frequency. This branched pulse train is
Further, most of the 48 mW pulse train is split into two by the split mirror 103, and is incident on the optical fiber by the coupling lens 126 via the reflecting mirror 104. Of these, 25 mW was coupled to the optical fiber and propagated. Here, an optical fiber delay line 128 having a total group velocity dispersion D ₂ of zero by connecting a normal optical fiber 160 m and a dispersion shift optical fiber 40 m was used as an optical delay line. Total length 200m
This optical fiber delay line 128 provides a delay of M = 200 times the repetition period with respect to the above repetition frequency of 200 MHz. Since both of the above two types of optical fibers have a positive tertiary chromatic dispersion, the optical fiber delay line 128
Has a third-order chromatic dispersion, and as a result, the above-described pulse train having a width of 60 fs is directly transmitted to the optical fiber delay line 128.
Propagating through it undergoes significant deformation. To avoid this, an optical filter 127 having a transmission wavelength width of 2 nm is used as a waveform shaper at the input end of the optical fiber delay line 128, thereby increasing the width of a pulse incident on the optical fiber delay line 128 to 1.5 ps. .

As described above, the other 2 mW pulse train split by the split mirror 103 is incident on the optical fiber 124 by the coupling lens 123. In this example, in order to increase the degree of freedom in the arrangement of the devices, the pulse train that does not pass through the optical delay line 128 is also a short (1 m) optical fiber 1.
The light propagates through the signal path 24 and is guided to the cross-correlator 107.

The 1 mW pulse train emitted from the optical fiber 124 and the 1 mW pulse train emitted from the delay line 128 are:
After being converted into parallel rays by the collimating lenses 125 and 129, the rays enter the cross-correlator 107. In the cross-correlator 107 used in the present example, as a non-linear effect, 2% of lithium niobate (LiNbO ₃ )
The harmonic generation effect is used, and the second harmonic light generated from the crystal is converted into a photomultiplier tube (not shown) in the cross-correlator 107.
And converted to a voltage value by a current amplifier (not shown) having an amplification sensitivity of 5 × 10 ⁴ V / A.
In this case, since 4 V was obtained as the maximum value of the cross-correlation signal voltage output from the cross-correlator 107, the reference voltage 108
Is set to 2 V, the difference from the cross-correlation signal voltage is applied to the integration circuit 109, and the output of the integration circuit 109 is transmitted through the drive circuit 110 to the displacement mirror 12 in the laser resonator 100.
Returned to 2.

FIG. 6 is a diagram showing characteristics obtained by the first embodiment. FIG. 6A shows the cross-correlator 107.
FIG. 6 is a diagram showing a result of measuring an autocorrelation waveform of a pulse incident on
The solid line indicates the result of the pulse emitted from the optical fiber 124, and the broken line indicates the result of the pulse emitted from the optical fiber delay line 128. From this, the width of the pulse emitted from the optical fiber 124 is estimated to be 100 fs, and the incident pulse width 60
clearly spread from fs. This is the optical fiber 1
The tertiary chromatic dispersion of 24 suggests that the pulse was deformed even by a propagation of only 1 m. On the other hand, for the pulse propagated through the optical fiber delay line 128 after the width has been increased to 1.5 ps using the waveform shaper (optical filter 127), the pulse width changes even after propagation of 200 m. Absent. This clearly shows the effectiveness of providing a waveform shaper before the optical delay line.

FIG. 6B is a diagram showing the phase noise characteristic measured for the laser light source whose repetition is stabilized by the first embodiment. Self-running C without stabilization
The phase noise power spectrum of an r-doped YAG laser is
This is indicated by the broken line in FIG. 8 which has already been used in the description of the conventional example in this specification. FIG. 8B shows a phase noise power spectrum in a state where the stabilization according to the first embodiment is performed in FIG.
In comparison with the self-running state shown by the broken line, it can be seen that in this example, the suppression of the repetition frequency fluctuation is achieved in all the frequency regions. Further, the deterioration of the phase noise reduction rate in the overtone of the power supply frequency as in the conventional example is not conspicuous, and an almost smooth phase noise power spectrum is obtained.

Here, a comparison is made between the optical pulse source according to the first embodiment of the present invention, in which the same laser light source is stabilized, and the optical pulse source according to the prior art, in terms of the price of a device added for stabilization. Let's try this. The most expensive part in the conventional example is the reference oscillator 532 in the example of FIG. 7, which actually accounts for 70% of the price of the additional device. This is followed by a high-speed photodetector 530, an amplifier 531 and a drive circuit 510, each accounting for 20%, 5% and 5% of the price. The prices of the other parts are almost negligible. On the other hand, in the first embodiment of the present invention, each of the optical filter 127, the cross-correlator 107, and the drive circuit 110 has substantially the same price as the drive circuit 510, and the others can be ignored. As a result, in this embodiment, an optical pulse source can be realized at a cost of only 15% of the conventional example.

As described above, the apparatus of this embodiment can realize an optical pulse source in which the short pulse characteristics of the light source are utilized, which is inexpensive, and whose repetition frequency is small with less inflow of noise by an electronic circuit.

(Second Embodiment) A second embodiment corresponding to the configuration of the second embodiment of the present invention described above with reference to FIG. 2 will be described below.

The configuration other than the cross-correlator 107 is the same as that of the first embodiment shown in FIG. 5, and the 1 mW pulse train emitted from the optical fiber 124 and the 1 mW pulse train emitted from the optical fiber delay line 128 are as follows. After being converted into parallel rays by the collimating lenses 125 and 129, the rays enter the cross-correlator 107. In the cross-correlator 107 used in this example, a two-photon transition in a silicon PIN photodetector is used as a nonlinear effect. In this silicon photodetector, λ _g is 1.1 μm and wavelength λ is 1.55
Equation (6) described above is satisfied for the optical pulse train of μm. The photocurrent generated from the cross-correlator 107 was converted to a voltage value by a current amplifier having an amplification sensitivity of 1 × 10 ⁷ V / A. In this case, the cross-correlation signal voltages output from the cross-correlator 107 have constants G ₀ and G ₁ of 150, respectively.
mV, 15mV, also because 15mV is obtained as the maximum value of G _c, by setting the reference voltage 108 to 173MV, by applying a difference between the cross-correlation signal voltage to the integrating circuit 109, the drive circuit and the output 110 Through the mirror 122.

According to the second embodiment, the above-mentioned first embodiment can be used.
The characteristics of FIG. 6 were obtained in the same manner as in the example of FIG. Regarding the price of the device portion added for stabilization, the cross-correlator 107 of the second embodiment is less than about half the price of the cross-correlator used in the above-described first embodiment. As a result, the second embodiment can be performed at a cost of only 12% of the conventional example.

Therefore, according to the device of the second embodiment, an optical pulse source having a low repetition frequency and a low repetition frequency can be realized by utilizing the short pulse characteristics of the light source, making it inexpensive, and reducing the inflow of noise by the electronic circuit. Was.

[0101]

As described above, the optical pulse source having a stabilized repetition frequency according to the present invention is not limited by the band of the high-frequency electronic circuit. Both the repeatability favors stabilization and there is no upper limit on the applicable repetition frequency. Further, the optical pulse source of the present invention having a stabilized repetition frequency can be realized at a low cost, and the inflow of noise by an electronic circuit is small, so that it can be expected that a great effect can be obtained industrially.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing an optical pulse source in which a repetition frequency is stabilized in a first embodiment of the present invention.

FIG. 2 is a configuration diagram showing an optical pulse source according to a modified example of the first embodiment of the present invention.

FIG. 3 is a configuration diagram showing an optical pulse source having a stabilized repetition frequency according to a second embodiment of the present invention.

FIG. 4 is a configuration diagram showing an optical pulse source according to a modification of the second embodiment of the present invention.

FIG. 5 is a configuration diagram showing an optical pulse source having a stabilized repetition frequency according to the first embodiment of the present invention.

6A and 6B show characteristics obtained by the first embodiment of the present invention, wherein FIG. 6A is a graph showing an autocorrelation waveform of a pulse incident on a cross-correlator, and FIG. 6B is a graph showing phase noise characteristics. is there.

FIG. 7 is a configuration diagram showing a conventional optical pulse source with a stabilized repetition frequency.

FIG. 8 is a graph showing phase noise characteristics obtained by a conventional example.

[Explanation of symbols]

 REFERENCE SIGNS LIST 100 laser resonator 101 resonator length adjustment mechanism 102, 103 branch mirror 104 reflecting mirror 105 waveform shaper 106 optical delay line 107 cross-correlator 108 reference voltage 109 integration circuit 110 drive circuit 111 displacement unit 112 oscillator 113 phase detection amplifier 114, 115 Reflecting mirror 116 Right angle reflecting prism 117 Converging lens 118 Nonlinear crystal 119 Aperture 120 Condensing lens 121 Photodetector 122 Displacement mirror 123 Coupling lens 124 Optical fiber 125 Collimating lens 126 Coupling lens 127 Optical filter 128 Optical fiber 129 Collimating lens 500 Laser resonance Instrument 502 Branch mirror 504 Reflecting mirror 509 Integration circuit 510 Drive circuit 522 Displacement mirror 530 High-speed photodetector 531 High-frequency amplifier 532 Reference oscillator 533 High-frequency mixer 534 Low-pass filter

Claims (15)

[Claims] 1. An optical pulse source comprising: a laser resonator for generating an optical pulse train; and a mechanism for adjusting an optical length of the resonator, wherein the optical length is adjusted to stabilize a repetition frequency of the optical pulse train. Branching means for branching an optical pulse train generated from the laser resonator; delay means for providing a non-zero integer multiple of a pulse repetition period to one of the optical pulse trains branched by the branching means; Optical means for incidentally focusing both of the other light pulse train as it is and the light pulse train delayed by the delay means into a medium having an optical non-linear effect; and generating the light pulse train in the medium having the non-linear effect. Converting means for converting a non-linear signal into an electric signal; and a driving system for driving a mechanism for adjusting an optical length of the laser resonator based on the electric signal. Optical pulse source characterized by comprising a means. 2. The optical pulse source according to claim 1, wherein said drive control means drives a mechanism for adjusting an optical length of said laser resonator so that said electric signal has a constant value. 3. The drive control unit includes: a comparison unit that compares the electric signal output from the light detection unit with a predetermined reference voltage; and based on an output signal of the comparison unit, the electric signal is compared with the reference voltage. 3. The optical pulse source according to claim 1, further comprising: driving means for driving a mechanism for adjusting an optical length of the laser resonator so as to match. 4. The method according to claim 1, wherein the medium having the nonlinear effect is a nonlinear crystal exhibiting a second harmonic generation effect or a material exhibiting a two-photon absorption effect, and the converting means is a light detecting means.
4. The optical pulse source according to claim 1, wherein the medium having the non-linear effect and the converting means are combined by one semiconductor photodetector exhibiting a two-photon absorption effect. 5. An optical pulse source according to claim 1, wherein said delay means is an optical fiber. 6. The optical pulse source according to claim 5, further comprising a waveform shaping unit configured to shape a pulse waveform at an incident end of the optical fiber. 7. A periodic delay means for providing a delay which changes periodically before giving a delay of a non-zero integral multiple of the pulse repetition period to a pulse train giving a non-zero integral multiple of the pulse repetition period. And a signal extracting means for extracting only a component in phase with the period of the periodically changing delay in the electric signal, wherein the drive control means controls the resonance of the resonator based on the in-phase component. The optical pulse source according to claim 1, wherein a mechanism for adjusting an optical length is driven. 8. A periodic delay means for providing a periodically varying delay to the other pulse train which does not provide a delay of a non-zero integral multiple of the pulse repetition period; Signal extracting means for extracting only a component having the same phase as the cycle of the delay to be performed, and driving the mechanism for adjusting the optical length of the resonator by the drive control means based on the component having the same phase. The optical pulse source according to claim 1, wherein 9. The periodic delay means comprises an oscillator and a displacement mirror connected to the oscillator, wherein the electric signal output from the light detection means has the same phase as the period of the periodically changing delay. 9. The optical pulse source according to claim 7, wherein the signal extracting means for extracting only the component is a phase detection amplifier connected to the oscillator. 10. An optical pulse source according to claim 7, wherein said delay means is an optical fiber. 11. The optical pulse source according to claim 10, further comprising a waveform shaping means for shaping a pulse waveform at an input end of said optical fiber. 12. The medium having the nonlinear effect is a nonlinear crystal exhibiting a second harmonic generation effect or a material exhibiting a two-photon absorption effect, and the converting means is a light detecting means, or the medium having the nonlinear effect is a light detecting means. 12. The medium according to claim 7, wherein said medium and said converting means are combined by one semiconductor photodetector exhibiting a two-photon absorption effect.
The light pulse source according to any one of the above. 13. A laser resonator for generating an optical pulse train, a resonator length adjusting mechanism for adjusting an optical length of the laser resonator, and a first branch for splitting an optical pulse train generated from the laser resonator A mirror; a second branching mirror for further branching one of the optical pulse trains branched by the first branching mirror; and a non-zero integer of a pulse repetition period to one of the optical pulse trains branched by the second branching mirror. An optical delay line for providing a double delay, a waveform shaper connected to an input end of the optical delay line for shaping a pulse waveform of an optical pulse train input thereto, and the other of the other portions still split by the second split mirror Both the optical pulse train and the optical pulse train delayed by the optical delay line are incident and focused on a medium having an optical non-linear effect, and a non-linear signal generated in the medium having the non-linear effect is converted into an electric signal. A cross-correlator, an integration circuit for comparing the electric signal output from the cross-correlator with a reference voltage, and the resonator based on an output signal of the integration circuit so that the electric signal matches the reference voltage. An optical pulse source, comprising: a drive circuit for driving a length adjustment mechanism. 14. The cross-correlator has an optical non-linear effect on both the optical pulse train of the other optical pulse train that has been split by the second splitting mirror and the optical pulse train delayed by the optical delay line. A reflecting mirror, a right-angle reflecting prism, and a converging lens as optical means for focusing into a medium; a nonlinear crystal exhibiting a second harmonic generation effect as a medium having the nonlinear effect; and a nonlinear signal generated by the nonlinear crystal. 14. The optical pulse source according to claim 13, further comprising a stop, a condenser lens, and a photodetector for converting the light into an electric signal. 15. The cross-correlator has an optical non-linear effect for both the optical pulse train of the other optical pulse train that has been split by the second split mirror and the optical pulse train delayed by the optical delay line. A two-photon absorber which also functions as a reflecting mirror, a right-angle reflecting prism, and a focusing lens, which are optical means for incident focusing on the medium, and a medium having the nonlinear effect and a means for converting a nonlinear signal generated in the medium into an electric signal. 14. The optical pulse source according to claim 13, further comprising a semiconductor photodetector exhibiting an effect.