JPS62274787A - Superhigh speed optical pulse generator - Google Patents

Abstract

PURPOSE:To enable regulation of the amount of deviation of oscillating wavelength per unit time, namely the chirp speed of the front output optical pulse of a semiconductor laser by a method wherein the optical pulse injected from the rear of the semiconductor laser is delayed by time nearly corresponding to the reciprocal of frequency of a driving signal, and return efficiency of the optical pulse to return to the semiconductor laser is made as variable. CONSTITUTION:A return efficiency variable circuit 3 is an optical circuit enabled to vary return efficiency to return again to a semiconductor laser 1 of an optical pulse injected from the semiconductor laser 1, namely a circuit enabled to vary transmissibility of light, for example. When time T of the optical pulse injected from the of the semiconductor laser l up to return again to the semiconductor laser 1 is equalized to the reciprocal of driving frequency (f) of a driving circuit 4, namely when the optical pulse injected from the semiconductor laser 1 is returned synchronizing with driving frequency (f), the chirp speed of the optical pulse varies corresponding to return efficiency of light. The chirp speed means the amount of deviation of wavelength when wavelength of light varies continu ously in the range within pulse width at pulse type optical output.

Description

[Detailed Description of the Invention] 3. Detailed Description of the Invention [Object of the Invention] (Field of Industrial Application) This invention is a method for generating ultra-high-speed optical pulses used as a light source in ultra-high-speed optical transmission, optical information processing, etc. This invention relates to an ultrahigh-speed optical pulse generator using a semiconductor laser.

(Prior Art) Conventionally, methods for generating optical pulses include an active mode locking method, a passive mode locking method, a gain switch method, a Q-switch method, a cavity dump method, and the like. Among these, optical pulse generation methods using semiconductor lasers are effective in terms of equipment scale, configuration, reliability, etc. in the high repetition frequency region of around 1 GHz or higher, which is aimed at application to ultra-high-speed optical transmission and optical information processing. It is. Methods for generating simple and high-speed optical pulses using a semiconductor laser include the gain switch method and the active mode periodic method.

The gain switch method uses a drive circuit 10 as shown in FIG.
In this method, an ultrashort optical pulse 103 is generated from the distributed feedback semiconductor laser 101 by directly applying a short current pulse with a large amplitude of several hundred picoseconds or less outputted from the distributed feedback semiconductor laser 101 to the distributed feedback semiconductor laser 101 . This method has the advantage that it has a relatively simple configuration and can generate a high-speed optical pulse train with a repetition frequency of several GH2 or more.

The active mode periodic method uses a drive circuit 10 as shown in FIG.
a semiconductor laser 10 driven by an output current pulse from 2;
1, a reflective surface 203 is provided for the laser diode 101, and the reflective surface 203 and one end surface 101- of the semiconductor laser 101 constitute an external resonator. This method generates a (1/1r) Hz optical pulse train 204 by modulating the repetition frequency. This method can generate pulses as short as 1 picosecond.

(Problems to be Solved by the Invention) In the gain switch method, the spectral width of the optical pulse is broadened due to wavelength chirping caused by fluctuations in the carrier density of the active layer, and the influence of dispersion is reduced during propagation through a long-distance optical fiber. As a result, the pulse width of the optical pulse during fiber propagation fluctuates, which causes problems such as deterioration of the extinction ratio, S/N ratio, etc. at the receiving section in optical fiber transmission, etc. Another problem is that it is difficult to generate optical pulses of 10 picoseconds or less due to the limited recovery time (about -7 picoseconds).

In addition, the active mode locking method can generate short pulses of about 1 picosecond as described above, but in order to actually generate this as a stable optical pulse with a uniform waveform, it is necessary to use a semiconductor laser. Complete anti-reflection (AR) coating is applied to the other end facet of 10j to eliminate extra Fabry-Perot modes such as those formed on both end faces of the semiconductor laser, and to increase the degree of coupling between the external cavity and the semiconductor laser as high as possible. However, it is necessary to form a stable external resonator mode, etc., and it is not easy to satisfy these conditions in an actual device, which is a problem when configuring as ii.

Therefore, as mentioned above, chirping control applicable to high-speed optical transmission and optical information processing is possible, and the pulse width is 10
There is a need to develop a new technology using semiconductor lasers that generate picosecond optical pulses with a repetition frequency of several GH@2 and below the picosecond.

The present invention has been made in view of the above, and an object thereof is to provide an ultrahigh-speed optical pulse generator capable of chirping control and variable pulse width control.

[Structure of the invention]

(Means for Solving the Problems) The ultrahigh-speed optical pulse generator of the present invention includes a drive circuit that generates a drive signal of a predetermined frequency, and a distributed feedback drive that generates an optical pulse by being driven by the drive signal of the drive circuit. type semiconductor laser, a delay means for delaying a light pulse emitted from the back surface of the semiconductor laser by a time approximately corresponding to the reciprocal of a predetermined frequency of the drive signal and returning it to the semiconductor laser; The present invention further comprises a return efficiency variable means for adjusting the deviation amount of the oscillation wavelength per unit time of the front output light pulse of the semiconductor laser by varying the return efficiency of the light pulse returned to the semiconductor laser.

(Function) In the ultrahigh-speed optical pulse generator of the present invention, the return efficiency of the optical pulse emitted from the back surface of the semiconductor laser is delayed by a time approximately corresponding to the reciprocal of the frequency of the drive signal, and the return efficiency of the optical pulse is varied. By doing so, it is possible to adjust the deviation of the oscillation wavelength per unit time of the front output optical pulse of the semiconductor laser, that is, the chirp speed.

(Example) Hereinafter, the present invention will be explained in detail using the drawings.

FIG. 1 is a diagram showing the basic configuration of an ultrafast optical pulse generator according to an embodiment of the present invention. The ultrafast optical pulse generator shown in the figure uses a distributed feedback semiconductor laser 1. The semiconductor laser 1 is driven by a drive signal such as a short current pulse with a repetition frequency r or a sine wave current with a frequency f output from a drive circuit 4, and by superimposing the drive signal on an appropriate bias current, and the gain A single-longitudinal mode optical pulse with a repetition frequency of 10 picoseconds (1 S) is generated using the principle of the switching method. The optical pulse is reflected by the reflecting surface 2 and returned to the semiconductor laser 1 via the return efficiency variable circuit 3.The return efficiency variable circuit 3 allows the light pulse emitted from the semiconductor laser 1 to return to the semiconductor laser 1 again. This is an optical circuit that can vary the return efficiency, for example, a circuit that can vary the light transmittance.A light pulse is emitted from the back surface of the semiconductor laser 1,
If the time T until re-injection is equal to the reciprocal of the drive frequency f of the drive circuit 4, that is, if the optical pulse emitted from the semiconductor laser 1 is returned in synchronization with the drive frequency r, the chirb velocity of the optical pulse becomes It was found that it changes depending on the return efficiency.

It should be noted that the chilb speed is the period of change when the wavelength of light changes continuously within the pulse width in pulsed light output. That is, it is the ratio of the pulse width to the difference between the wavelength at the leading edge of the pulse and the trailing edge. FIG. 2 is a graph showing an example of the relationship between return efficiency η and chilb speed. From this graph, if the return efficiency η is varied from 100 (no reinjection) to about -10 dB, the chirp speed will be from 5 x 10-4 nm/l+3 to 100 dB.
It can be seen that it can be varied up to a range of -3n/ps.

As described above, the variable return efficiency circuit 3 is disposed between the semiconductor laser 1 and the reflective surface 2, and the time required for the optical pulse driven by the drive circuit 4 and emitted from the semiconductor laser 1 to return to the semiconductor laser 1. When T is set equal to the reciprocal of the drive frequency of the drive circuit 4, the chirve speed of the optical pulse emitted from the semiconductor laser 1 can be changed by varying the return efficiency of the variable return efficiency circuit 3.

FIG. 3 is a diagram showing another basic configuration of an ultrafast optical pulse generator according to another embodiment of the present invention.

In this embodiment, the configurations of the semiconductor laser 1, the reflective surface 2, the variable return efficiency circuit 3, and the drive circuit 4 are the same as those in FIG. A certain quasi-dispersive optical fiber 6, ie, a single mode fiber (SMF), is newly provided, and the optical pulse emitted from the front surface of the semiconductor laser 1 passes through this optical fiber 6 and is output. This optical fiber 6 has a normal dispersion with respect to the oscillation wavelength of the semiconductor laser 1, and by varying the return efficiency by the return efficiency variable circuit 3, the output light pulse 5 is outputted through the optical fiber 6. The width can be changed. To explain in more detail, in a normally dispersive medium, light on the long wavelength side has a short medium transit time, that is, a short delay time. In addition, since the emitted light pulse 5 from the semiconductor laser 1 is a light pulse having a red shift zirbing in which the wavelength shifts to a longer wavelength with time, when it passes through the optical fiber 6 which is a normal dispersion medium, it oscillates with a delay. The long-wavelength light that oscillated quickly catches up with the short-wavelength light that oscillated earlier, and the pulse is compressed, resulting in a shorter light pulse. FIG. 4 is a graph showing an example of a change in pulse width with respect to the amount of dispersion of an optical fiber.
This is for a 6 nm/ps optical pulse, and the curve shows a chirb velocity of approximately 0. This is for an optical pulse of 015 nm/ps. As shown in Figure 4, the amount of dispersion of the optical fiber was selected to be about 80 ps/nm, and the chirp speed was changed from 0.026rv/ps to 0.015'nm.
/ps, the pulse width of the output optical pulse can be changed from about 40 ps to about 8 ps. It should be noted that before entering the optical fiber, the pulse width of the optical pulses having two different Chirb velocities becomes narrowest at a dispersion amount that is approximately the reciprocal of the Chirb velocities, and the pulse width at this time is approximately several ps. Therefore, by varying the chirp speed of the front-emitted optical pulse 5 of the semiconductor laser 1, that is, by varying the return efficiency of the variable return efficiency circuit 3, the output optical pulse from the optical fiber 6 having a constant amount of dispersion can be produced. This allows the pulse width to be changed. In the above embodiment, the optical fiber 6 was used as the normal dispersion substance, but the present invention is not limited to this.

FIG. 5 is a block diagram of an ultrafast optical pulse generator according to still another embodiment of the present invention. This ultrafast optical pulse generator is a more detailed configuration of the ultrafast optical pulse generator having the basic configuration shown in FIGS. 1 and 3. In this ultra-high speed optical pulse generator, the distributed feedback semiconductor laser 'f12 is deeply modulated by a high frequency current having a frequency r of several GHz or more from the drive circuit 11, and the pulse width is approximately several 10 at a repetition frequency r. Generates picosecond light pulses. This optical pulse causes red shift chirping in which the oscillation wavelength shifts several A to the longer wavelength side with time. This shift value per unit time is the chirp speed.

The optical pulses emitted from the back side of the semiconductor laser 12 are coupled to the optical fiber 14 via the focusing lens 15 and the aperture 19 which is return efficiency variable means. The optical fiber 14 is a light return optical fiber having a reflective coating with a high reflectance on the far end 14a.

Therefore, the optical pulse incident on the optical fiber 14 is reflected by the far end surface 14a and re-enters the semiconductor laser 12 via the aperture 19 and the focusing lens 15.

At this time, similarly to the embodiment shown in FIG.
? If the length or length of the feedback optical fiber 14 is set so that the frequency of the sine wave current of the driving circuit 11 to be driven and the time taken for the optical pulse to emit the semiconductor laser 12 and re-enter the semiconductor laser 12 are equal. By adjusting the diaphragm 19 and varying the return efficiency for re-entering the semiconductor laser 12, it is possible to change the chirp speed of the output light pulse to the output optical fiber 18. That is, when calculating the delay time due to the optical fiber 14, the delay time t due to the length of the optical fiber 14 is t=1.5, where the speed of light is C.
1, and since the optical pulse from the semiconductor laser 12 travels back and forth through the optical fiber 14, the delay time due to the optical fiber 14 is 3T110.

Ignoring the delay caused by the focusing lens 15 and assuming that the frequency of the drive signal is f, the required length of the optical fiber 14 is 1. t
f (mm) = 100/I (GH2). Now, for example, when driving at 5 GHz, optical fiber 1
The length history of 4 should be 20IIIll.

A light pulse emitted from the front surface of the semiconductor laser 12 is transmitted through a coupling lens 13, an optical isolator 16, and a convergence lens 17.
is supplied to the output optical fiber 18 with high efficiency.

If the above-mentioned high dispersion optical fiber having a high normal dispersion value is used as the optical fiber 18, the pulse width of the output light pulse can be adjusted according to the return efficiency by the aperture 19, as described above. Pulse width is several 1
The chirb speed of a 0 picosecond light pulse is 10'n1
ll/ps, so it ranges from several picoseconds to several tens of seconds.
To vary the pulse width in the picosecond range, the fraction m
This means that it is sufficient to use an optical fiber 18 whose speed is approximately several tens of picoseconds/r+m. For example, the normal variance value is 20
If a ps/nld optical fiber 18 is used, a length of approximately 1-51 volts may be selected.

In this manner, in the embodiment shown in FIG. 5, the pulse width of the output light pulse can be adjusted by adjusting the aperture diameter of the aperture 19 and varying the return efficiency.

FIG. 6 shows another embodiment of the invention. This embodiment differs greatly from the embodiment shown in FIG. 5 in that a return focusing rod lens 22 is used instead of the return optical fiber 14. That is, in this embodiment, the distributed feedback semiconductor laser 21 is driven by a drive signal from the drive circuit 26 as in the embodiment shown in FIG. Focusing type rod lens 2 through
The light is reflected by a light reflecting surface 23 provided near the far end surface of the semiconductor laser 21, and is again injected into the semiconductor laser 21.

Further, the optical pulses emitted from the front surface of the semiconductor laser 21 are supplied to an output or compression optical fiber 25 via a coupling focusing rod lens 24.

Since the length of the feedback focusing rod lens 22 in the optical axis direction is usually several mm, there is a delay time during which the light pulse emitted from the semiconductor laser 21 is reflected by the light reflecting surface 23 and returns to the semiconductor laser 21 again. is approximately 100 pS. Therefore, in this embodiment, it is useful that the driving frequency of the driving circuit 26 is about 10 GHz or more. Note that the feedback focusing rod lens 22 can be slightly moved in the direction of its tip by means of a screw 31, and by this fine movement, the return efficiency of the optical pulse passing through the feedback focusing rod lens 22 can be varied. This allows the chirp speed or pulse width of the output optical pulse to be adjusted as in the case described above.

〔Effect of the invention〕

As explained above, according to the present invention, since the return efficiency of the optical pulse emitted from the semiconductor laser is returned with a time delay approximately corresponding to the reciprocal of the frequency of the drive signal, the pulse width can be changed. It outputs optical pulses of several tens of picoseconds and a repetition frequency of several GH7 or more, and the chirping speed of the optical pulses can be continuously adjusted. In addition, by compressing the optical pulse from the front output of the semiconductor laser and applying pulse compression technology using Chirb compensation, the pulse width of the optical pulse can be changed from a few picoseconds depending on the return efficiency. It can be adjusted continuously over a range of several tens of picoseconds, and is useful for ultra-high-speed optical transmission, optical information processing,
It can be expected to be applied to optical sampling, optical gate elements, etc.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the basic configuration of an ultrahigh-speed optical pulse generator according to an embodiment of the present invention, FIG. 2 is a graph showing the relationship between chirb speed and return efficiency, and FIG. 3 is a diagram showing another embodiment of the present invention. FIG. 4 is a graph showing the relationship between the pulse width and the amount of dispersion of the optical fiber, and FIG. 5 is a diagram showing another basic configuration of the ultrafast optical pulse generator according to the example, and FIG. FIG. 6 is a block diagram of an ultrafast optical pulse generator according to another embodiment of the present invention. FIGS. 7 and 8 are block diagrams of conventional high-speed optical pulse generators, respectively. 1.12.21... Semiconductor laser 2... Reflection surface 3... Return efficiency variable circuit 6.18.25... Optical fiber 14... Optical fiber 19... Aperture 22... Feedback Focusing rod lens Fig. 1 Return efficiency '7 [del Fig. 3 Dispersion amount (ps/nm) Fig. 4 Fig. 5b Fig. 6 Fig. 7

Claims (3)

[Claims] (1) A drive circuit that generates a drive signal of a predetermined frequency, a distributed feedback semiconductor laser that is driven by the drive signal of the drive circuit to generate an optical pulse, and a dispersion feedback semiconductor laser that generates an optical pulse emitted from the back surface of the semiconductor laser. a delay means for returning the light pulse to the semiconductor laser with a time delay approximately corresponding to the reciprocal of a predetermined frequency of the drive signal; and a delay means for returning the light pulse to the semiconductor laser after being delayed by the delay means; and a return efficiency variable means for adjusting the deviation amount of the oscillation wavelength per unit time of the front output optical pulse. (2) The semiconductor laser has normal dispersion for compressing the optical pulse from the front output so that the pulse width of the optical pulse from the front output can be adjusted according to the return efficiency varied by the return efficiency variable means. 2. The ultrahigh-speed optical pulse generator according to claim 1, further comprising a magnetic medium means on the front side of the laser. (3) The ultrahigh-speed optical pulse generator according to claim 2, wherein the normal dispersion medium means is constituted by an optical fiber.