JPH0637380A - Semiconductor short pulse light source and generating method of short light pulse train - Google Patents

Abstract

PURPOSE:To generate a short light pulse train which has a frequency positive- integer-times as high as the frequency of a modulation signal by a method wherein one or more gain modulation regions or absorption modulation regions are periodically provided with specific position relations in an optical resonator. CONSTITUTION:A laser which is used for ultra-high optical communication has gain modulation regions 9 which change an optical gain in regard to a time and passive waveguides 25 and 26 in its optical resonator. One or a plurality of the gain modulation regions 9 are provided along a route through which a light travels in the optical resonator in such a manner that an optical length which the light travels since it starts from the gain modulation region till it reaches the adjacent or the same gain modulation region is one positive-integer' th as short as an optical length which the light travels to make a round trip in the optical resonator. With this constitution, the light is made to pass through a plurality of the gain modulation regions 9 while it makes a round trip in the optical resonator and a light pulse train having a higher frequency than an external modulation frequency can be generated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor short pulse light source for generating a short optical pulse synchronized with an electric signal and a method for generating an optical pulse train thereof, which is necessary for ultra high speed optical communication of 10 GHz or more. is there.

[0002]

2. Description of the Related Art In current optical communication, semiconductor laser light is directly modulated or modulated by a semiconductor laser to form signal light, which is then input to an optical fiber. With this method, an optical signal up to about 20 Gbit / sec can be obtained, but high-speed transmission beyond that exceeds the modulation band of the laser and the modulator, and cannot be modulated. As a method of overcoming this limit, as shown in FIG. 13 (a), an optical signal by a short optical pulse is formed by a plurality of gain switches of semiconductor lasers 1 and this is gradually increased by a delay line 2 using an optical fiber. There is a way to combine after delaying. According to this method, by increasing the number of semiconductor lasers, the frequency of the signal can be increased until the light pulses overlap. However, when the pulse interval becomes narrow due to the multiplexing, the variation in the pulse interval due to the error in the delay time of each pulse becomes a problem.

Therefore, as shown in FIG. 13 (b), the multiplexed signal light 3 ', the clock pulse light 3 by the mode-locked laser 1'and the AND element 2' (Kerr effect of the non-linear material etc. are utilized. ), The pulse interval becomes constant and the pulse width can be narrowed.

By the way, the conditions required for this clock pulse light source are (1) that the pulse interval and pulse width are narrow, and (2) that they are synchronized with the signal light. Although this clock pulse light source can be synchronized with the signal light even if it is a short pulse light source that self-oscillates, it is easier to use the short pulse light source driven by a high frequency electric signal from an external circuit. A pulse is obtained.

The generation of a short optical pulse train is performed by a laser provided with a gain medium and a waveguide in an optical resonator, or by a structure based on this structure which is improved for pulse narrowing such as addition of saturable absorption. Performed using. The medium of the laser may be solid, gas or liquid, and various lasers have been used to generate an optical pulse by mode locking. Recently, research on mode-locked lasers using semiconductor lasers has been actively conducted for the purpose of application to optical communication, and especially monolithic mode-locking in which components such as a gain region and a waveguide are integrated on one device. Lasers are receiving attention.

FIG. 14 is a schematic view of the most basic mode-locked laser, which comprises an optical gain region 4 and a passive waveguide 5 provided on an InP semiconductor substrate 33. The optical gain region 4 and the passive waveguide 5 are arranged in series in the Fabry-Perot resonator formed by both crystal end faces (light reflecting faces) 6. Reference numeral 7 is an active layer in the optical gain region, 37 is a p-electrode, and 38 is an n-electrode.

The active layer 7 in the optical gain region has a wavelength of 1.5 μm.
InGaAsP, and the core 8 of the optical waveguide has a wavelength of 1.
3 μm InGsAsP. The generation of the optical pulse train is
The light generated in the optical gain (amplification) region 4 is realized by modulating the gain of the optical amplification region 4 with a high frequency current having a cycle equal to the round-trip time in the optical resonator. With this method, the width is 4.4p
S. Tucker et al., Electronics Letters Vol.25 p.6, that a short pulse train with a repetition frequency of 40 GHz was obtained.
Announced on 21.

The principle of mode locking will be described below by taking this element as an example. Since this element is basically a Fabry-Perot laser, many longitudinal modes occur during laser oscillation. If the interval of each longitudinal mode is expressed by the frequency of light,
It is almost equal to the reciprocal of the round-trip time of light in the resonator. In the following discussion, not only the intensity of each longitudinal mode is focused, but each longitudinal mode is treated as an electromagnetic field component that changes with the frequency of light. Now, when the gain of the optical amplification region 4 is modulated at a period equal to the round-trip time in the optical resonator (round trip time, the reciprocal of the round trip time is the round trip frequency), the sidebands generated from each longitudinal mode are on both sides. It occurs in the immediate vicinity of the longitudinal mode, and this generated sideband wave is coupled with the adjacent longitudinal modes. The sidebands originate from all longitudinal modes, so that each longitudinal mode becomes coupled through the sidebands. As a result, the phases of the longitudinal modes are aligned. Furthermore, when longitudinal mode coupling does not occur, the longitudinal mode spacing is slightly different due to the refractive index dispersion. However, when the longitudinal modes are coupled, the frequency intervals of the longitudinal modes also become constant, and the intensities of many longitudinal modes become uniform. This state is called mode-synchronized. Since the phase of each longitudinal mode is random at the time of no modulation, the intensity of the laser light (proportional to the square of the electric field) determined by the sum of each longitudinal mode is slightly random near a certain constant value with respect to time. It just changes.

However, when the mode locking as described above occurs, the phases of the longitudinal modes are aligned and the frequency intervals are constant, so that the intensity of the laser light determined by the sum of the longitudinal modes is clear from the Fourier series theorem. In this way, the reciprocal of each longitudinal mode interval, that is, the gain modulation period, changes periodically. Moreover, since many longitudinal modes are coupled, the light intensity is pulsed, as can be seen by a simple calculation. This is the principle of mode synchronization.

[0010]

As described above, in the generation of an optical pulse, the frequency of the optical pulse train matches the modulation frequency, so that the optical pulse train is limited by the band of the external drive circuit, etc. In the high frequency region of 10 GHz and above, the change over time of the gain of the semiconductor laser decreases as the modulation frequency increases, so that the semiconductor laser light source itself reaches the frequency limit even if the band of the external drive circuit increases.

In order to solve the above problems, an object of the present invention is to provide a semiconductor short pulse light source for generating an optical pulse train having a higher repetition frequency than the conventional one while synchronizing with an external circuit, and a method for generating the optical pulse train. It is in.

[0012]

In order to solve the above-mentioned problems, according to the present invention, one or more gain modulation regions or absorption modulation regions are periodically arranged in an optical resonator, and light is an optical resonator. Generates an optical pulse train with a frequency higher than the external modulation frequency by passing through the optical gain modulation region or the optical absorption modulation region multiple times at equal time intervals before returning to the original position after making one round trip or one round trip inside Is.

[0013]

[Operation] One or more gain modulation regions or absorption modulation regions are periodically arranged in the optical resonator. By modulating the light traveling back and forth in the optical resonator as a cycle, the modulation frequency The present invention will be described in detail below with reference to examples, which are capable of generating light pulses of a natural multiple.

[0014]

FIG. 1 shows the principle of the present invention for generating a frequency-doubled optical pulse train. Its structure is such that a gain modulation region 9 is arranged in the center of an optical resonator, and It passes through the gain modulation region twice during one round trip.
In the figure, 10 and 11 are light reflecting surfaces, 21 is a core of an optical waveguide,
26 is a passive waveguide, 33 is an InP substrate, 34 is n-type In
P cladding layer, 35 is p-type InP cladding layer, and 36 is p
InGaAs cap layer, 37 p electrode, 38 n electrode, 39 InGsAs / InGaAsP quantum well active layer, and 40 non-doped optical waveguide layer.

In the mode-locked laser having this structure, when the gain modulation region or the absorption modulation region is modulated with a period of time during which light reciprocates in the resonator as a cycle, an optical pulse train having twice the modulation frequency is generated. The principle will be described based on FIG.

Since light is an electromagnetic wave, that is, a wave, its electric field strength or magnetic field strength can be expressed by a wave function.
Now, the coordinate axis x is set along the path along which light propagates in the optical resonator. That is, the resonator length is L, and the light reflection surface 10 on the left side is
If the coordinate of x is 0, the coordinate 11 of the right light-reflecting surface is x
= L, the coordinates when returning to the left light reflection surface 10 again is x = 2L. When t is time, the electric field strength E (t, x) is expressed by the following equation.

E (t, x) = A (ωt−kx) sin (ωt−kx) where ω is the angular frequency of light (2πf, f is the frequency of light), and k is the wavenumber of light (2πN). / Λ, N is the refractive index), and A (X) is a function representing the envelope of the electric field. Time t
The origin of is selected so that the electric field can be described by the above functional form. (Ωt-kx) is the phase φ of light, and by using the phase φ, the electric field strength E (t, x) can be expressed by a univariate function as in the following equation.

E (φ) = A (φ) sin (φ) The modulation received by the equiphase surface of light having a certain phase φ during one round trip in the optical resonator is examined. The change in the gain of the gain modulation region 9 is sinusoidal with respect to time (FIG. 2 (a), sinf
_m tf _m : modulation frequency), the period of which corresponds to the transit time of the light in the resonator, the phase of the modulation signal (when the equiphase surface of the light exits the gain modulation region and returns again) f _m
When t) is deviated by π (FIG. 2B), the gain modulation that is received when the light travels to the right (outward path) and the gain modulation that is received when the light returns to the left (return path) are shown. ). Therefore, the sum of the modulation of the gains received during one round trip in the resonator is 0, and the light is not modulated. However, the situation is different when the change in gain is distorted and not sinusoidal. FIG. 3 is an explanatory diagram in the case where the injection current is half-wave rectified by utilizing the rectifying property of the double heterojunction and the time change of the gain is made non-sinusoidal. FIG. 3 (a) shows the time variation of the gain. As is clear from this figure, the sum of modulations per round trip is no longer 0,
Moreover, as shown in FIG. 3 (b), φ is modulated twice. Therefore, when observing the electric field strength at a specific position, for example, at the point of x = 0, light is modulated twice in one reciprocating period in the resonator, that is, in one modulation period. Therefore, the sidebands generated in each of the longitudinal modes are generated in the immediate vicinity of the two longitudinal modes, and the longitudinal modes are coupled with each other. As a result, a light pulse having twice the modulation frequency is generated. In addition to utilizing the rectifying property of the double heterojunction to distort the gain, nonlinear phenomena of various semiconductor lasers such as nonlinearity due to interaction between laser light and injected carriers and gain nonlinearity with respect to injected carrier concentration are used. Can be used.

Although the case of modulating the gain has been described above, the optical absorption region and the optical amplification region are provided in the optical resonator, and the absorptance of the optical absorption region is periodically changed at the round trip frequency to obtain the optical absorption region. Mode locking can be realized even by modulating the light generated and amplified in the amplification region. In addition, as an example of modulation of the absorptance, when an electric field is applied to a semiconductor or a quantum well, the absorptance changes due to the Franz-Keldesh effect or the quantum confined Stark effect, respectively. With these effects, the change in absorption is not proportional to the applied voltage. Therefore, when a voltage of a predetermined magnitude is applied to the semiconductor layer or the quantum well layer, a large strain occurs in the change in absorption rate with time. Therefore, the generation of mode-locking using absorption modulation enables the same operation as the case of using the rectifying property of the gain modulation region. There are n modulation regions in which the modulation regions of the embodiment of FIG. 1 are arranged at equal intervals, and when a modulation signal is simultaneously input to each region without being delayed, an optical pulse train having a frequency of 2n times is generated. This is easy to understand.

Furthermore, the modulation period, even if no match to one round trip time of light in the resonator may be a natural number (k) times the round trip time t _r. This can be explained as follows. Light between modulation period kt _r is an optical resonator reciprocates k times. During this time, the light shuttling in the resonator passes through the 2nk times gain modulation region and are observed at some point result, the light going through the point is modulated 2nk times during the modulation period kt _r ing. Therefore, an optical pulse train having a frequency of 2nk / kt _r is generated. Modulation frequency f _m is 1 / kt _r
Therefore, the period f _p of the optical pulse train is the modulation frequency f _m
2kn times [(2nk / kt _r ) ÷ (1 / kt _r )].

[0021] adding a saturable region element having n modulation area described above, (n is the number of modulation region) 2n times the pre-round-trip frequency f _r should undergo passive mode-locking in a cycle of Then, it becomes easy to generate an optical pulse. When the signal of the frequency f _r is input to the modulation region, by the action of n modulation region, the optical pulse train generated by the passive mode-locking is subjected to frequency modulation of 2nf _r. Strictly speaking,
The frequency of the passive mode-locked optical pulse train is 2n of the input signal.
It does not match twice, but by inputting the modulation signal,
The frequency of the optical pulse train is drawn to 2n times the modulation signal,
Optical pulse train frequency 2nf _r synchronized with the input signal will be produced.

The element as shown in FIG. 1 can be manufactured by the following procedure. First, an n-type InP cladding layer 34 of 1 μm, a non-doped optical waveguide layer 40 (wavelength 1.2 μm) of 0.1 μm, and an InGaAs / InGaAsP multiple quantum well are formed on an n-type InP substrate 33 by metal organic chemical vapor deposition. Active layer 39, non-doped optical waveguide layer 40 (wavelength 1.2 μm) 0.1 μm, p-type InP clad layer 35
Of 1 μm and the InGaAs cap layer 36 of 0.2 μm. After that, an SiO2 film is deposited on this growth layer, and the SiO2 film is left only in the region to be the gain modulation region 9 by using the photolithography technique and the chemical etching technique. Using this SiO2 film as a mask, the cap layer 36, the p-type clad, and the active layer 35 in the region to be the passive waveguide 26 are removed. After that, the core 21 of the optical waveguide and the non-doped cladding 41 are again formed by the metal organic growth method.
To grow. At this time, no semiconductor layer grows on the gain modulation region covered with the SiO2 film. Then, the SiO2 film covering the gain modulation region is removed to form the p electrode 37 and the n electrode 38. Finally, by cleavage, the light reflecting surface 10
By forming and 11, the device is completed.

FIG. 4 shows the structure of a semiconductor short pulse laser according to the second embodiment of the present invention, which can generate an optical pulse having a frequency four times the modulation frequency. 20
Is the active layer of the semiconductor laser, and 21 is the core of the optical waveguide. This device includes a gain modulation region 14 and a passive waveguide 2
6, a monolithic mode-locked laser composed of the refractive index modulation regions 16 and 16 'generated an optical pulse train of 4 times frequency. There are two gain modulation regions 14, and the position thereof (pointing to the center of the gain modulation region) is at a distance from the left reflecting surface 10 that is 1/3 and 3/4 of the total element length. Further, one refractive index modulation region 16 is provided between the gain modulation regions 14 and another
One refractive index modulation area 16 ′ is provided between the gain modulation area 14 and the left reflecting surface 10 and the right reflecting surface 11. Also,
The length of the refractive index modulation area 16 ′ near the light reflection surface is ½ of the length of the central refractive index modulation area 16. As a result, the light emitted from the gain modulation region 14 is always reflected by the adjacent or light reflecting surface (10 or 11) and returns to the same gain modulation region 14 again until the refractive index modulation region is 1 or less. Exist, and the distance that light passes through them is all equal. The layer structure of the gain modulation region 14 has a quantum well width of 1
InGaAs / InGaAs with 0 nm barrier layer width of 50 nm
It is a P quantum well layer. The number of quantum wells is 6 and InG
The wavelength of the aAsP barrier layer is 1.2 μm. The core 21 of the passive waveguide 26 is 1.3 μm InGsAsP.
The layer structure of the refractive index modulation regions 16 and 16 'is the same as that of the passive waveguide 26 except that it has electrodes. The element length is 2 mm, and the gain modulation region 14 is superposed with a direct current of 50 mA at 20 GHz (round trip frequency 20 GHz).
When a high frequency power of 0 dBm was applied, an optical pulse train of 80 GHz was generated. High frequency power at this time is 30 dB
Since m (maximum current amplitude 400 mA) is large, the injected current is half-wave rectified, the time change of gain is not sinusoidal, and the generation of an optical pulse having a quadruple frequency can be realized. By changing the DC voltage applied or the DC current injected to the refractive index modulation regions 16 and 16 ', the refractive index of this region is changed and the optical length of the element can be adjusted. As a result, the frequency at which mode locking occurs (pulse repetition frequency) can be adjusted.

The manufacturing procedure of this device is similar to that of the device of the first embodiment, and it suffices to change the mask for photolithography.

Other configurations and operations are similar to those of the first embodiment.

FIG. 5 shows a third embodiment of the present invention. A semiconductor laser structure is divided by an electrode 22, and a monolithic mode-locked laser used as a gain modulation region 14, an active waveguide 15, and a saturable region 17 is operated by operating an injection current or an applied voltage to generate a quadruple frequency optical pulse train. did. There are two gain modulation regions 14, and the position (pointing to the center of the gain modulation region) is 1/100 of the total element length from the left reflecting surface 20.
Points at distances of 4 and 3/4. The position of the saturable region 17 (which indicates the center of the saturable region) is the center of the two gain modulation regions 14. This position is also the center of the device. The element length is 2 mm and the gain modulation is 20 GHz.
When a high frequency power of 30 dBm (round trip frequency 20 GHz) was applied, an optical pulse train of 80 GHz was generated. The saturable region 17 has the effect of shortening the optical pulse width. Further, in the device shown in this figure, since the saturable region 17 is placed in the center of the gain modulation region 14, collision mode locking occurs and the optical pulse width becomes shorter.

The voltage applied to the saturable region 17 and the current injected into the active waveguide 15 are adjusted to cause passive mode locking at a frequency four times the round trip frequency, that is, 80 GHz. In this state, the gain modulation area 14 has 20
When high frequency power of 25 dBm is input at 8 GHz,
An optical pulse train having a frequency of 0 GHz continued to be generated, and the optical pulse was synchronized with the input high frequency power.

In the manufacturing method of this element, first, the same semiconductor laser structure as that of the first embodiment is grown by the metal organic growth method, thereafter the p electrode 22 and the n electrode 38 are formed, and the light reflecting surface is formed by cleavage. Good.

Other configurations and operations are similar to those of the first embodiment.

FIG. 6 shows a fourth embodiment of the present invention. A semiconductor laser structure is divided by an electrode 22, and a monolithic mode-locked laser used as a gain modulation region 14, an active waveguide 15, and a saturable region 17 is operated by operating an injection current or an applied voltage to generate a quadruple frequency optical pulse train. did.
In the structure of this element, the number and position of the gain modulation regions 14 are the same as those in the second embodiment, but the saturable region 17 is placed in contact with the optical anti-slope. The element length is 2 mm (round trip frequency 20 GHz). The voltage applied to the saturable region 17 and the current injected into the active waveguide 15 are adjusted to cause passive mode locking at a frequency four times the round trip frequency, that is, 80 GHz. When high-frequency power of 20 GHz and 25 dBm was input to the gain modulation region 14 in this state, an optical pulse train having a frequency of 80 GHz continued to be generated, and the optical pulse was synchronized with the input high-frequency power.

The manufacturing method of this element is in accordance with the third embodiment.

Other configurations and operations are similar to those of the first embodiment.

FIG. 7 shows a fifth embodiment of the present invention. The semiconductor laser structure is divided by the electrode 22 and the injection current is manipulated to generate a double frequency optical pulse train in the monolithic mode-locked laser used as the gain modulation region 14 and the active waveguide 15. The element length is 2 mm, the number of the gain modulation region 14 is one, and it is in the center of the device (the center of the gain modulation region 14 coincides with the center of the device). In this gain modulation area 14, 20 GHz (round trip frequency 20 GHz
Hz), 30 dBm of high frequency power applied, 4
A 0 GHz optical pulse train was generated. Even if the frequency is modulated at 10 GHz, which is half the round trip frequency, 4
A 0 GHz optical pulse train was generated.

When a voltage below the build-in potential of the double heterojunction forming this element is applied to the gain modulation area 14 of the element of FIG. 7 and a high-frequency voltage is superposed on it, this area is absorbed and modulated. Can work as. This is because the absorption coefficient of the active layer in this region changes depending on the electric field due to the Franz-Keldesh effect or the quantum confined Stark effect (when the active layer is a quantum well). When the gain modulation region 14 is operated as the absorption modulation region, the light traveling in the element receives the gain in the active waveguide 15 and is absorbed in the absorption modulation region, but the absorption amount is periodically generated by the high frequency voltage. Mode synchronization is achieved because of the change to. Unless the amount of change in the absorption coefficient is proportional to the strength of the high frequency voltage, an optical pulse train having a frequency that is a natural multiple of the modulation frequency is generated as in the case of gain modulation. Such non-linearity is obtained by increasing the amplitude of the applied high frequency voltage. Also in this case, the relationship between the cycle of the generated optical pulse and the cycle of the applied high frequency voltage is the same as that in the case of gain modulation.

The manufacturing method of this element is in accordance with the third embodiment.

Other configurations and operations are similar to those of the first embodiment.

FIG. 8 shows a seventh embodiment of the present invention. It is clear that the absorption modulation region 25, the optical amplification region 23, and the passive waveguide 26 in the center also generate an optical pulse having a double frequency due to the absorption modulation operation. In FIG. 8, both the absorption modulation region 25 and the optical amplification region 23 are realized by using the same semiconductor laser structure. The structures of the absorption modulation region 25 and the optical amplification region 23 are the same as the gain amplification region 14 of the first embodiment, and the structure of the optical waveguide 26 is the same as the optical waveguide 26 of the first embodiment.

The manufacturing method of this element is in accordance with the first embodiment.

Other configurations and operations are similar to those of the first embodiment.

FIG. 9 shows the reproduction of an optical clock using the semiconductor optical pulse light source according to the present invention.
In the generation of optical pulses (due to mode-locking) of the present invention, the pulses are formed after the light has traveled back and forth through the resonator multiple times.
Moreover, the modulation received per round trip need not be large. Therefore, a pulse train is generated without necessarily undergoing modulation each time it passes through the modulation region, and a pulse train is generated even when the modulation signal is coded. Now, consider an element having n modulation regions. Let f _{r be the} round trip frequency and f _{s be} the clock frequency of the coded signal. Now, setting the cavity length such that f s ₌ 2nf _r, for example, (2n-1) between the light even if missing number component signal to make one round trip in the resonator undergoes once modulated. Therefore, it is easy to understand that the optical pulse train is generated from the coded signal and its period corresponds to f _s . still,
generation of light pulse even if the f _s to a natural number times of 2nf _r is clearly possible. Using this principle, as shown in FIG. 9, the transmitted coded optical signal 18 is once detected by the photodetector 12, and the electric signal 27 amplified by the amplifier 30 is used for the element according to the present invention, that is, the mode-locked laser 13 Is modulated, an optical signal 19 synchronized with the optical signal is generated, and an optical clock can be generated.

[0041] Now, with elements saturable region is added as shown in FIG. 5 or FIG. 6, the left causing the passive mode-locking frequency in advance 2nf _r, the optical clock recovery is facilitated. In such a state, since passive mode locking has already occurred, when a coded signal having a frequency close to this repetition frequency is input to the modulation area, the frequency of the optical pulse train becomes the frequency of the input signal. Is drawn in and the optical clock is regenerated. The input power required at this time is smaller than that when there is no saturable region, and the optical pulse is easily synchronized with the input signal even if the interval in which the optical pulse does not come is widened.

When the elements of Examples 1 to 6 were inserted as the mode-locked laser 13 shown in FIG. 9, an optical pulse was reproduced. The principle of reproducing the optical pulse is as described above. When the mode-locked laser is used in the gain modulation operation, the electric signal generated by the photodetector 12 is amplified and then a constant current is biased. When the absorption modulation operation is performed, the photodetector 1
After amplifying the electric signal generated in 2, the constant voltage below the Hilton potential is biased. In the case of the element of FIGS. 4 and 5, the element length was set to 4 mm (round trip frequency 10 GHz). A coded optical signal 18 having a repetition frequency of 40 GHz is input, and a repetition frequency of 4
0 GHz electrical signal 27 (minimum holding time t ^h 28 above the discrimination level and minimum t holding time below the discrimination level t
¹ 29 reciprocal of the sum is 40GHz) of the photodetector 12
When formed by the amplifier 30 and input to the mode-locked laser 13, an optical pulse train 19 having a period of 40 GHz was generated. In the case of the elements of FIGS. 7 and 8, the element length remains 2 mm (round trip frequency 20 GHz), and in the case of the element of FIG. 6, the element length is lengthened to 4 mm (round trip frequency 10 GHz) and the passive mode is set. Repetition frequency of 40G with 40GHz optical pulse being generated in synchronization
When the optical signal 18 coded in Hz was input, an optical pulse train 19 having a repetition frequency of 40 GHz was generated in synchronization with the input signal. Moreover, in the element of FIG. 5, the element length is 4 m.
m, and when a coded optical signal 18 having a repetition frequency of 40 GHz is input in the state where a 40 GHz optical pulse is generated by passive mode synchronization, an optical pulse train 19 having a repetition frequency of 40 GHz is generated in synchronization with the input signal. did.

It should be noted that even if the repetition frequency of the coded signal in all the above examples is doubled, an optical clock having the same frequency as this repetition frequency was reproduced.

The optical clock recovery circuit shown in FIG. 9 has no inductance component other than the bias coil 31 and is therefore easy to integrate. FIG. 10 is a conceptual diagram of an opto-electronic-integrated-circuit (OEIC) in which the circuit of FIG. 9 is integrated.

As described above, the timing is extracted by using the resonance characteristic of the mode-locked laser. In addition to this method, timing extraction is performed using the resonance characteristic of an electric circuit, and the mode-locked laser (including the mode-locked laser according to the present invention) or the optical clock can be regenerated even when the laser is driven by the extracted electric signal. Obviously it is possible. FIG. 11 shows the LC circuit 3 for timing extraction.
FIG. 3 is a conceptual diagram of an optical clock recovery circuit using 2; The resonance frequency of the LC circuit 32 is adjusted to the frequency of the input signal.

As shown in FIG. 12, a modulation region 41 made of non-doped InP cladding is newly provided at the end of the device according to the present invention (FIG. 12 is based on the device of FIG. 7), and this newly provided modulation is provided. If only the region is modulated with the round trip frequency, an optical pulse train having a frequency matching the round trip frequency is generated. Therefore, the same element can generate an optical pulse train having two frequencies without changing the frequency of the driving high frequency power.

Other configurations and operations are similar to those of the first embodiment.

Although the present invention has been specifically described based on the embodiments, the present invention is not limited to the above embodiments, and it goes without saying that various modifications can be made without departing from the scope of the invention. Yes.

[0049]

As described above, according to the present invention, it is possible to generate a short optical pulse train having a frequency that is a natural multiple of the frequency of the modulated signal. Also, the optical clock can be regenerated from the coded optical signal.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of an embodiment of a mode-locked laser according to the present invention.

FIG. 2 is an explanatory diagram of the operation of the mode-locked laser according to the present invention.

FIG. 3 is an explanatory view of the operation of the mode-locked laser according to the present invention.

FIG. 4 is an explanatory view of another embodiment of the mode-locked laser according to the present invention.

FIG. 5 is an explanatory view of another embodiment of the mode-locked laser according to the present invention.

FIG. 6 is an explanatory view of another embodiment of the mode-locked laser according to the present invention.

FIG. 7 is an explanatory view of another embodiment of the mode-locked laser according to the present invention.

FIG. 8 is an explanatory diagram of another embodiment of the mode-locked laser according to the present invention.

FIG. 9 is a conceptual diagram of optical clock recovery according to the present invention.

FIG. 10 is a conceptual diagram of an OEIC for optical clock recovery.

FIG. 11 is an explanatory diagram of an optical clock recovery circuit using an LC circuit.

FIG. 12 is an explanatory view of another embodiment of the two-frequency mode-locked laser according to the present invention.

FIG. 13 is an explanatory diagram of an optical communication system using short optical pulses.

FIG. 14 is an explanatory diagram of a conventional mode-locked laser.

[Explanation of symbols]

9 ... Gain modulation area, 10 ... Left light reflection surface, 11 ... Right light reflection surface, 12 ... Photodetector, 13 ... Mode-locked laser according to the present invention, 14 ... Gain modulation area, 15 ... Active waveguide, 1
6 ... Central refractive index modulation area, 16 '... Refractive index modulation area near light reflection surface, 17 ... Saturable area, 18 ... Coded optical signal, 19 ... Regenerated optical clock, 20 ... Semiconductor laser Active layer, 21 ... Optical waveguide core, 22 ... Electrode,
23 ... Optical amplification region, 25 ... Absorption modulation region, 26 ... Passive waveguide, 27 ... Coded signal converted from optical signal to electric signal, 28 ... Minimum holding time above discrimination level, 29 ... Minimum below discrimination level Hold time, 30 ... Electric amplifier, 3
1 ... Ink conductance for bias, 32 ... LC circuit for timing extraction, 33 ... InP substrate, 34 ... n-type In
P clad layer, 35 ... p-type InP clad layer, 36 ... p
Type InGaAs cap layer, 37 ... P electrode, 38 ... N electrode, 39 ... InGsAs / InGaAsP quantum well active layer, 40 ... Non-doped optical waveguide, 41 ... Non-doped In
P-clad.

 ─────────────────────────────────────────────────── --- Continuation of the front page (72) Inventor Koji Nonaka 1-1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Inside Nippon Telegraph and Telephone Corporation (72) Takashi Kurokawa 1-1-6 Uchisaiwai-cho, Chiyoda-ku, Tokyo Nippon Telegraph and Telephone Corporation

Claims (8)

[Claims] 1. A laser having a gain modulation region for changing the gain of light with time and a passive waveguide or an active waveguide in an optical resonator, wherein the gain is along a path along which the light travels in the optical resonator. A single or a plurality of modulation regions are arranged, and the optical length that the light travels from the time when the light leaves the gain modulation region until the light reaches the next adjacent or the same gain modulation region is that the light travels within the optical resonator. A semiconductor short pulse light source characterized by being a natural fraction excluding one of the optical length of one round trip to return to the original position. 2. A laser having an absorption modulation region for changing the absorptance of light with time and a gain region having a constant gain with respect to time in an optical resonator, or a laser in which an optical waveguide is added to the laser. , A single or a plurality of absorption modulation regions are arranged along a path along which light travels in the optical resonator, and one of the optical lengths until the light makes one round trip in the optical resonator and returns to the original position A semiconductor short pulse light source characterized by being a natural fraction excluding. 3. The semiconductor short pulse light source according to claim 1, wherein the optical resonator has a saturable absorption region. 4. Each element constituting an optical pulse light source is made of a semiconductor double heterojunction, and an electrode is provided at a double heterojunction between gain modulation regions or a double heterojunction between absorption modulation regions to inject a current. 4. The semiconductor short pulse light source according to claim 1, wherein the optical length between the gain modulation region or the absorption modulation region can be changed by changing the refractive index by applying a voltage. . 5. The semiconductor short pulse light source according to any one of claims 1 to 4, wherein the light absorption rate of the gain or absorption modulation area of the gain modulation area is calculated by making one round trip of light in the optical resonator. A method for generating an optical pulse train, which is characterized in that the time until returning to the position is periodically changed with a cycle. 6. A semiconductor short pulse light source according to any one of claims 1 to 4, wherein the light is cyclically changed with a cycle of a natural number multiple, excluding one time of one round trip of light in the optical resonator. And a method for generating an optical pulse train. 7. The optical pulse using any one of the semiconductor short pulse light sources according to claim 1, wherein the input signal to the gain modulation region or the absorption modulation region is a binary coded electrical signal The time for one round trip of light in the resonator of the light source is the minimum duration of the high voltage or current state of the encoded electrical signal and the minimum duration of the low voltage or current state of the encoded electrical signal. In addition, an optical pulse train is generated by using the optical pulse light source having an element length that matches the time obtained by multiplying the sum of the number of the gain modulation regions or the absorption modulation regions of the optical pulse by two. Method of generating optical pulse train. 8. The optical pulse light source using the semiconductor short pulse light source according to claim 1, wherein the input signal to the gain modulation region or the absorption modulation region is a binary coded electrical signal. The time for one round trip of light in the resonator is the sum of the minimum duration of the high voltage or current state of the encoded electrical signal and the minimum duration of the low voltage or current state of the encoded electrical signal. And an optical pulse train using the optical pulse light source having an element length that matches a natural number multiple except one of the time obtained by multiplying twice the number of the gain modulation region or the absorption modulation region of the optical pulse light source. A method for generating an optical pulse train, which comprises: