JPH08195713A - Optical clock pulse generating circuit - Google Patents

Abstract

PURPOSE: To obtain an optical clock pulse S2 whose clock frequency is an integer multiple n.f of a clock frequency (f) of an optical data signal string S1. CONSTITUTION: An optical clock extract element 1 has an optical resonator consisting of a saturable absorption region 11 and a gain region 12 and whose length L is about c/(2nηf)}, where (c) is the velocity of light and (η) is a group speed refractive index of light, and is in the mode synchronization operation. When an optical data signal string S1 is made incident from an end face 20a of the optical resonator, the frequency of the optical pulse generated from the element 1 in the mode synchronization operation is locked to the frequency n.f and an optical clock pulse S2 whose clock frequency is n.f is emitted from an end face 20b of the optical resonator. Furthermore, the saturable absorption region 11 prevents spread and missing of the optical pulse circulating through the optical resonator.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention directly responds to an optical data signal train having a clock frequency of f to obtain a clock frequency of n.multidot.f.
The present invention relates to an optical clock pulse generation circuit that generates an optical clock pulse (n is an integer of 2 or more).

[0002]

2. Description of the Related Art In ultra-high-speed optical communication, an optical clock pulse is directly fed from an optical data signal train to an optical signal processing circuit in order to avoid the processing speed limitation when converting an optical signal into an electric signal and performing signal processing. There is a pressing need to generate.

A conventional optical clock pulse generation circuit of this type is disclosed in Japanese Patent Laid-Open Publication No. 6-13981 (Title of Invention: Optical Timing Extraction Circuit). This optical clock pulse generation circuit uses a passive mode-locking semiconductor laser whose optical propagation path is composed of a saturable absorption region and a gain region as an optical clock pulse extraction element, and uses an optical clock pulse (bit rate) M of light. An optical clock pulse having a clock frequency M / m (m is a natural number) is obtained from the data signal train. That is, the optical clock pulse generation circuit obtains an optical clock pulse obtained by dividing the clock frequency M at a rate of a natural number from an optical data signal sequence having a clock frequency M.

[0004]

In the above-mentioned conventional technique, only an optical clock pulse having a clock frequency equal to or lower than the clock frequency of the original optical data signal train can be obtained.

However, in the optical communication system, when the optical data signal sequence is multiplexed and transmitted, it is necessary to generate an optical clock pulse having a higher clock frequency than the original optical data signal sequence.

[0006]

In the optical clock pulse generation circuit of the present invention, an optical data signal train having a clock frequency f is made incident on a semiconductor laser device and the clock frequency n · f (n is 2 or more) is supplied from the semiconductor laser device. (Integer of) an optical clock pulse generation circuit for emitting an optical clock pulse,
The semiconductor laser device has substantially the same length {c / (2nη
f)} (where c is the speed of light and η is the refractive index of the group velocity of light) is provided with a resonator having an optical propagation path, one end of the resonator being the incident end of the optical data signal train, and The other end is the emission end of the optical clock pulse.

One of the optical clock pulse generation circuits is
The optical propagation path of the resonator may have a saturable absorption region for light and a gain region for the light.

Another one of the optical clock pulse generation circuits can be configured such that the optical propagation path of the resonator includes only a gain region of light.

[0009]

Next, the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram of a first embodiment according to the present invention. The optical clock extraction element 1 in the embodiment is shown in a detailed sectional view. FIG. 2 is a timing chart of the first embodiment.

The optical clock pulse generation circuit of this embodiment will be described with reference to FIGS.
1 (see FIG. 2) is an end face 2 of an optical clock extraction device 1 using a mode-locked semiconductor laser device via an optical fiber 2a, a lens 3a, an optical isolator 4a and a lens 3b.
It is incident on 0a. The optical data signal sequence S1 is a return-to-zero code signal subjected to intensity modulation of "1" and "0". The clock frequency of the optical data signal sequence S1 is f
And The optical clock extraction element 1 has a length L of approximately {c /
(2nηf)} (where c is the speed of light and η is the group velocity refractive index of light) and is provided with a resonator having an optical propagation path. In this resonator, the light propagation path is composed of a saturable absorption region 11 of light and a gain region 12 of light, and end faces 20a and 20b are respectively the resonator ends of the resonator. The optical clock extraction element 1 emits an optical clock pulse S2 from the end face 20b. The optical clock pulse S2 has a clock frequency of n ·
It is a pulse train of f. In FIG. 2, the clock frequency of the optical clock pulse is shown as 2f, that is, n = 2. The optical clock pulse S2 is guided to the optical fiber 2b via the lens 3c, the optical isolator 4b, and the lens 3d.

Here, the lenses 3a and 3b are used to efficiently make the optical data signal sequence S1 emitted from the optical fiber 2a incident on the end face 20a of the optical clock extracting element 1, and the lenses 3c and 3d are the optical clock extracting element. The optical clock pulse S2 emitted from the first end face 20b is used to efficiently enter the optical fiber 2b. Also,
The optical isolator 4a is the end surface 20a of the optical data signal sequence S1.
The optical isolator 4b removes the returning light of the optical clock pulse S2 from the optical fiber 2b by absorbing the returning light from the optical fiber 2b, and stabilizes the operation of the device 1.

Next, the optical clock extraction element 1 will be described in detail. A semiconductor laser device having a quantum well is used as the optical clock extraction device 1. The element 1 has a compound semiconductor as a main material, and has a laser oscillation wavelength of 0.8 μm, for example.
Combination of GaAs and AlGaAs for band, combination of GaAs and InGaAs for wavelength 0.98 μm band, In for 1.3 μm band and 1.55 μm band
A combination of P and InGaAsP is preferred. Element 1
Is a substrate 17 having a crystal structure, an n-clad layer 16, an active layer 15, a p-clad layer 14 and a cap layer 13a, 1
3b are arranged in layers. A p-side electrode 20a is provided on the cap layer 13a, a p-side electrode 20b is provided on the cap layer 13b, and an n-side electrode 19 is provided on the substrate 17. That is, it should be noted that the p-side electrodes 20a and 20b are separated. In addition, the end surface 20 that is a surface perpendicular to the active layer 15
Reference symbols a and 20b are cleavage planes of the element 1 having a crystal structure.

The active layer 15 is composed of the n-clad layer 16 and the p-layer.
The refractive index is higher than that of the cladding layer 14, and the end faces 20a and 20a
It forms a main propagation path of light (optical data signal train S1 and optical clock pulse S2) entering and exiting from 0b. Since the end faces 20a and 20b are the cleavage planes of the crystal, some of the light passes through and some of the light is reflected. Therefore, the end face 20a and the active layer 1
The light propagation path mainly composed of 5 and the end face 20b constitute a resonator for laser oscillation light. In the element 1 of this example, the length L of this resonator is approximately {c / (2nη) as described above.
f)}. The end faces 20a and 20b may be coated to adjust the reflection coefficient of light. The cap layer 13a is an ohmic layer that connects the p-side electrode 18a and the p-clad layer 14, and the cap layer 13b is an ohmic layer that connects the p-side electrode 18b and the p-clad layer 14.

In the optical clock extraction element 1, the n-side electrode 19 is set to the ground potential, the p-side electrode 18a is applied with a negative voltage, that is, reverse bias, and the p-side electrode 18b is applied with a positive voltage, that is, forward bias. A current is injected from the p-side electrode 18b to the n-side electrode 19. Then, the light propagation path facing the p-side electrode 18 a becomes the saturable absorption region 11 of light, and the p-side electrode 18 a
The light propagation path facing b is the light gain region 12. In the saturable absorption region 11, absorption of light with high intensity is saturated and absorption of light with low intensity is absorbed. The degree of absorption of this light changes depending on the reverse bias voltage and the like. On the other hand, gain region 1
In 2, the passing light is amplified, and when the injection current into the gain region 12 becomes larger than the threshold value, the element 1 starts laser oscillation, and when the optical data signal sequence S1 is not incident, the repetitive pulse frequency becomes An optical pulse train of approximately 2f is applied to the end face 20a.
And 20b.

The semiconductor laser in the above state is a so-called mode-locked semiconductor laser. One of the bias conditions that causes the mode-locked operation is, for example, a laser oscillation wavelength of 1.5.
When the resonator length L of the device 1 is about 4,100 μm and the saturable absorption region 11 is about 200 μm in the 5 μm band, the injection current from the p-side electrode 18b is 184 mA and the p-side electrode 18a is 18 a.
The reverse bias voltage to is 0.4V. At this time, an optical clock pulse S2 having a clock frequency (repetition frequency) of about 10.7 GHz is obtained. Since the group velocity refractive index η of light changes depending on the reverse bias applied to the p-side electrode 18a and the injection current from the p-side electrode 18b, even if the clock frequency n · f of the optical clock pulse S2 is the same,
The physical resonator length L of the element 1 needs to be changed according to these conditions. On the contrary, even if the physical length of the resonator of the device 1 is constant, the propagation path length of light is changed by changing the bias applied to the p-side electrodes 18a and 18b,
A properly mode-locked optical clock pulse S2 can be generated.

FIG. 3 is a diagram for explaining the operation principle of the first embodiment.

The operation of this embodiment will be described below with reference to FIGS.

The optical clock extraction element 1 is set with operating conditions such as bias so as to be mode-locked. Then, even when the optical data signal sequence S1 is not incident, the element 1
Is the time taken for the light to circulate in the resonator (hereinafter referred to as resonator circulation time) T / n (= 1 / (n · f), n = 2 in FIGS. 2 and 3, and hereinafter n = 2) Corresponding to, an optical pulse having a repetition frequency of 2f is generated.

Under the above operating conditions, the optical clock extraction element 1 receives the optical data signal sequence S1 (data "1") having the clock frequency f (time interval T) from the end face 20a, which is one of the resonator ends, at time t0. Upon incidence, this optical data signal sequence S1 is transferred to the end face 2 which is another one of the resonator ends after the time T / 4.
It reaches 0b. This optical pulse generated by the signal train S1 is reflected by the end face 20b and reaches the end face 20a after time T / 2 from time t0. This light pulse is reflected by the end face 20a and travels toward the end face 20b, as if at time t0.
Optical data signal sequence S1 of data "1" after time T / 2 from
Acts as if incident on element 1. Since the optical pulse continuously circulates in the resonator of the element 1 as described above, even if the data “0” continues for a long time in the optical data signal sequence S1,
The repetition time T / 2 when viewed from the end faces 20a and 20b.
It seems that the optical data signal sequence S1 of (repetition frequency 2f) exists.

Since the end faces 20a and 20b not only reflect but also pass the light pulse, the light pulse is emitted from the element 1 from the end face 20b as the optical clock pulse S2 having the repetition frequency 2f. For example, when the laser oscillation wavelength is 1.55 μm band, the repetition frequency is 50 G
The optical data signal train S1 of Hz has a resonator length L of about 440 μm.
When it is incident on the element 1 having the element, a 100 GHz optical pulse train S2 is emitted from the end face 20b of the element 1.

Here, the saturable absorption region 11 of the optical clock extraction element 1 suppresses spontaneous emission light by the absorption saturation effect of high intensity light and the absorption effect of low intensity light, and the optical pulse is circulated within the resonator. It plays the role of preventing the pulse width from spreading or disappearing. Therefore, the length of the saturable absorption region 11 is preferably set so that the half width of the optical clock pulse S2 is the narrowest. The optical data string S2
The greater the intensity of, the smaller the timing jitter of the optical clock pulse S2 and the more stable the generation operation of the optical clock pulse S2.

The operation of the optical clock extraction device 1 described above is as follows.
It can also be explained in the frequency domain. That is, the optical data signal sequence S1
As can be seen from the Fourier transform, the clock frequency f and its integral multiple n are added to both sidebands of the optical center frequency f0.
-Including the frequency component of f. The resonator length L of the element 1 is selected so that the longitudinal mode frequency interval of light is approximately n · f. Therefore, when the optical data signal sequence S1 is incident on the element 1, the longitudinal mode frequency interval is n.multidot.
The clock frequency n is drawn in so that only the n · f frequency component is amplified in the resonator.
The optical clock pulse S2 of f is emitted from the element 1.

FIG. 4 is a block diagram of the second embodiment according to the present invention.

The optical clock pulse generation circuit of the second embodiment has the same optical fibers 2a and 2b, lenses 3a to 3d and optical isolators 4a and 4b as in the first embodiment, and the optical clock extraction circuit of the first embodiment. An optical clock extraction element 1A obtained by partially changing the configuration of the element 1 is used as a component. Element 1
A is a cap layer 13a divided in the element 1
And 13b and p-side electrodes 18a and 18b are integrated, respectively, and cap layer 13 and p-side electrode 18 are provided. Further, the n-side electrode 19 is set to the ground potential, and the p-side electrode 18
Is applied with a positive voltage to inject a current into the element 1A.
The injection current into the device 1A is a threshold value of laser oscillation or a current value higher than that. Therefore, this element 1A
Has only a gain region of light and no saturable absorption region of light. The length L from the end surface 20a to the end surface 20b of the element 1A is approximately equal to {c / (2
n η f)}.

In the present embodiment, the optical clock extraction element 1A, when the optical data signal sequence S1 is not incident,
The longitudinal mode frequency interval determined by the resonator length L is nf
The multi-mode light as shown below is emitted from the end faces 20a and 20b. The multimode light emitted at this time is continuous light because the phase intervals between the longitudinal modes are not uniform.
However, when the optical data signal train S1 of data "1" is incident on the end face 20a of the element 1, the multimode light is drawn into the clock frequency component of the optical data signal train S1 and the end face 20
From b, the optical clock pulse S2 of the clock frequency n · f
Is emitted. The optical clock extraction element 1A of this embodiment is
Although there is no action to prevent the pulse width of the optical clock pulse S2 from increasing due to the saturable absorption region, the optical clock pulse is provided under appropriate conditions such that the longitudinal mode frequency interval of light determined by the cavity length L matches n · f. The pulse width spread of S2 can be made sufficiently small. Also, the optical clock extraction element 1A
Has the advantage that the structure is simpler than the optical clock extraction element 1 used in the first embodiment.

[0027]

As described above, according to the present invention, the semiconductor laser device has substantially the same length {c / (2nηf)} (where c
Is a speed of light, n is an integer of 2 or more, and η is a resonator having an optical propagation path of a group velocity refractive index of light). There is an effect that it is possible to obtain an optical clock pulse having a clock frequency n · f in response to

[Brief description of drawings]

FIG. 1 is a configuration diagram of a first embodiment according to the present invention.

FIG. 2 is a timing diagram of the first embodiment.

FIG. 3 is a diagram for explaining the operation principle of the first embodiment.

FIG. 4 is a configuration diagram of a second embodiment according to the present invention.

[Explanation of symbols]

 1, 1A Optical clock pulse extraction element 2a, 2b Optical fiber 3a to 3d Lens 4a, 4b Optical isolator 11 Saturable absorption region 12 Gain region 13, 13a, 13b Cap layer 14 p Clad layer 15 Active layer 16 n Clad layer 17 Substrate 18, 18a, 18b p-side electrode 19 n-side electrode 20a, 20b End surface

Claims (3)

[Claims] 1. An optical clock pulse generation for inputting an optical data signal train of clock frequency f to a semiconductor laser device and emitting an optical clock pulse of clock frequency n · f (n is an integer of 2 or more) from the semiconductor laser device. In the circuit, the semiconductor laser device has a substantially length {c / (2nη
f)} (where c is the speed of light and η is the refractive index of the group velocity of light) is provided with a resonator having an optical propagation path, one end of the resonator being the incident end of the optical data signal train, and An optical clock pulse generation circuit, wherein the other end is an emission end of the optical clock pulse. 2. The optical clock pulse generation circuit according to claim 1, wherein the optical propagation path of the resonator includes a saturable absorption region of light and a gain region of the light. 3. The optical clock pulse generation circuit according to claim 1, wherein the optical propagation path of the resonator includes only a gain region of light.