JPS60141044A - Method of frequency modulation of semiconductor laser - Google Patents

Abstract

PURPOSE:To obtain a method of frequency modulation small in waveform deterioration due to the influences of frequency characteristic even in case of direct frequency modulation of a semiconductor laser by using mBnB code for a pulse modulation signal, and especially setting the value of (m) and (n) of the mBnB code to satisfy a specified limited condition. CONSTITUTION:An mBnB code is used for a pulse modulation signal, and especially, values of (m) and (n) are so set that the mBnB code satisfies conditions m<=9, 2<=n<=10, m<n. A semiconductor laser 3 is biased to a fixed level higher than an oscillation threshold value by the bias current from a bias circuit 1. On the other hand, a pulse modulation signal 4 is reduced to a minute modulation signal current 6 consisting of mBnB code in a code converting circuit 5 and added t the semiconductor laser 3, and a frequency modulation output 7 small in waveform deterioration is obtained. As the frequency modulation output waveform obtained by direct modulation of the semiconductor laser is high in modulation frequency, it goes to a differential waveform. But the wavedorm deterioration is suppressed to a relatively small value as shown in the figure.

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical field to which the invention pertains) The present invention relates to a method for frequency modulating output light of a semiconductor laser.

(Introduction of conventional technology) In recent years, the characteristics of semiconductor lasers have improved, and it has become possible to obtain semiconductor lasers that oscillate in a single-axis mode and have high spectral purity. has become possible. In particular, in the case of the FSK heterogain optical communication method that uses frequency information, it is possible to directly frequency modulate the output light of the semiconductor laser by minutely changing the injection current of the semiconductor laser, making it possible to easily create a system with low loss. It has the advantage of being configurable. This direct frequency modulation of a semiconductor laser has two effects: the carrier density in the laser medium changes due to changes in the injection current, which changes the refractive index of the laser medium, and the temperature of the laser medium changes in response to changes in the injection current. This is caused by two effects: the effect of changing the refractive index of . However, both of the above-mentioned effects have frequency characteristics, and the magnitude of frequency shift differs depending on the modulation frequency. Therefore, during pulse modulation, waveform deterioration occurs in response to the frequency characteristics described above.
There were six drawbacks: poor reception sensitivity. (Saito, Shiraki, Moku, “Coherent optical fiber transmission modification technology -
FSK Heterodyne Detection”, Practical Application Report, No. 31
Volume, No. 12) (Object of the Invention) An object of the present invention is to provide a frequency modulation method that causes less waveform deterioration due to the influence of the frequency characteristics even during direct frequency modulation of a semiconductor laser.

(Structure of the Invention) The present invention provides a frequency modulation method for frequency modulating the output light of a semiconductor laser by minutely changing the injection current of the semiconductor laser using a pulse modulation signal.
The f3 code is used, and the values of m and le are set so that the mB code satisfies the following conditions: 1≦m≦9.2≦ru≦10 and m<ru.

(Principle of the invention) Next, the principle of the present invention will be explained in detail using the drawings.

FIG. 1 is a diagram showing the modulation frequency dependence of the frequency shift of unit current υ of a semiconductor laser having a buried heterostripe (BH) structure. Further, FIGS. 2 and 3 are waveform diagrams when direct frequency modulation is performed on a semiconductor laser at low speed and high speed, respectively.

When the injection current of a semiconductor laser is slightly changed at a low frequency, the refractive index of the laser medium changes due to thermal fluctuations accompanying the change in the injection current, and the laser output light is frequency modulated. This thermal effect is greater as the modulation frequency is lower, as shown by the broken line in FIG. Therefore, when frequency modulation is performed using a low-speed pulse signal of 10 Mb/s or less, the output is subjected to the same effect as when it passes through a low-pass circuit, so the output waveform becomes an integral waveform.

FIG. 2(a) shows the modulation signal, and FIG. 2(b) shows the output waveform when frequency modulation is applied mainly due to thermal effects during low-speed modulation.

On the other hand, the injection current of the semiconductor laser is 100 MHz ~ IG
When the frequency is changed at a high frequency such as H, z, the refractive index within the laser medium changes due to variations in the carrier density within the laser medium, and the laser output light is subjected to frequency modulation. The effect of this carrier is shown by the solid line in FIG. 100MH
In the modulation frequency range from z to about IGHz, the higher the modulation frequency, the larger the frequency shift. Therefore, several hundred Mb/s
When frequency modulation is performed using a high-speed pulse signal, the output is subjected to the same effect as when it passes through a high-pass circuit, so the output waveform shows a differential waveform. FIG. 3(a) shows a modulated signal, and FIG. 3(f) shows an output waveform when frequency modulation is applied mainly due to carrier effects during K high-speed modulation.

As shown above, since the magnitude of the frequency shift is dependent on the modulation frequency, waveform deterioration occurs in the output waveform during frequency pulse modulation. As is clear from Figures 2 and 3, the waveform deterioration is greatest when the signal is continuous with ``1'' or ``0'', so if there is no continuous ``1'' or ``θ'', that is, the average output fluctuation. It is possible to reduce the above-mentioned waveform deterioration by using a code with a small value.The mBtLB code is a code obtained by converting an m-bit signal into a rub-bit signal larger than m bits.The mBrLB code is If used, 1 after sign conversion
It is possible to suppress "continuity" or "0" continuity to less than Lupitt, and it is possible to construct a signal sequence with small average output fluctuation.

(Y, Takasaki et al., “0ptic
al Pulse Formats for Fiber
0ptic Digi tal Communica
tions” IEFiE Trans.

coM, Mo1. C0M-24, No 4. Ap
Ril, 1976) In this case, by converting m bits to rubits, the required band becomes wider and the reception sensitivity deteriorates accordingly. Therefore, the optimal values of m, BnB code recruitment, and l can be determined from the magnitude of waveform deterioration and the deterioration caused by widening of the band. FIG. 4 is a diagram showing waveform deterioration, deterioration due to widening of the band, and deterioration in combination of both when a certain semiconductor laser having a BH structure is modulated with mB(ry++1)B code.

In this example, it can be seen that the amount of deterioration is minimum when m is 3. Since the waveform deterioration during general KFM modulation is quite large, the optimum m can be found within the range of 1≦m≦9. In other cases, that is, when the optimum m is 10 or more, the frequency characteristics during FM modulation are relatively small and waveform deterioration does not occur. Therefore, there is almost no improvement in sensitivity by converting to mBnB code.

Still, the value of n is generally set to about m-1-1 or m+2 in consideration of the band.

(Example 1) Next, the present invention will be explained in detail with reference to Examples.

FIG. 5 is a block diagram for explaining the first embodiment of the present invention, and FIG. 6 is a waveform diagram of each part of the first embodiment of the present invention.

In this embodiment, the semiconductor laser 3 is biased by a bias current 2 from a bias circuit 1 to a constant level above the oscillation threshold. On the other hand, the pulse modulation signal 4 is turned into a minute modulation signal current 6 consisting of mBnB code in the code conversion circuit 5 and is applied to the semiconductor laser 3, thereby obtaining a frequency modulation ratio cuff with little waveform deterioration. In this embodiment, a BH type semiconductor laser having frequency modulation characteristics as shown in FIG. 1 was used as the semiconductor laser 3.

The pulse modulation signal 4 consists of a 400 Mb/S N-Z signal, and the code conversion circuit 5 converts the code of 1" to 10".
By converting the 0" code into the 01" code, it is converted into a biphase code, which is a type of υIB2B code.

The frequency of the semiconductor laser 3 was modulated by the minute modulation signal current 6 consisting of this biphase code. The waveform of the pulse signal 4 at this time is shown in Fig. 6 (at), and the waveform of the minute modulation signal current consisting of a biphase code is shown in Fig. 6 (b). This allows frequency modulation obtained by directly modulating the semiconductor laser. Since the output waveform has a high modulation frequency, it becomes a differential waveform, but as shown in Figure 6(C), the waveform deterioration is kept relatively small.In the case of the semiconductor laser used in this example, When frequency modulation is performed directly on the signal, the reception sensitivity deteriorates by more than 7 dB during signal demodulation due to waveform deterioration, and it was not possible to achieve a code error rate of less than 10'', but converting to biphase code. Deterioration due to the band being doubled by doing so 3
The signal could be demodulated with a code error rate of 10-9 or less with a reception sensitivity degradation of 5 dB from the theoretical value, including dB.

(Embodiment 2) FIG. 7 is a block diagram for explaining a second embodiment of the present invention, and FIG. 8 is a waveform diagram of each part of the second embodiment of the present invention.

In this embodiment, the code conversion circuit 5 m 73 n
The minute modulation signal current 6 converted into the B code is further waveform-shaped by a compensation circuit 8 and then applied to the semiconductor laser 3. Here, the compensation circuit 8 is constituted by an electric circuit having frequency characteristics that compensate for the frequency characteristics during frequency modulation of the semiconductor laser. The other configurations are similar to the first embodiment. In this example as well, 400
If a compensation circuit for Mb/S pulse transmission is not inserted, the first
As shown in the embodiment, the output waveform shows a differential waveform.

Therefore, an integrating circuit was used as the compensation circuit 8. The output waveform of the compensation circuit 8 is shown in FIG. 8 (cl). Here, FIG. 8 (al is the waveform of the pulse signal 4. ) By frequency modulating a semiconductor laser with a waveform having an integral characteristic as shown in Fig. 8 fd+, we were able to obtain a modulated output with almost no waveform deterioration.
By combining the f1BWB code and an electrical compensation circuit, direct frequency modulation of a semiconductor laser with almost no waveform deterioration can be realized.

In addition to the above-described embodiments, various modifications can be made to the present invention. For example, as the semiconductor laser 3, it is possible to use various types of semiconductor lasers in addition to the BH type.

As the compensation circuit 8, it is possible to use ones with various characteristics depending on the signal transmission speed. For example, during low-speed modulation, since the output waveform shows an integral waveform, a distribution circuit can be used.

(Effects of the Invention) As described above, according to the present invention, frequency modulation optical communication with less waveform deterioration can be realized even during direct frequency modulation of a semiconductor laser.

[Brief explanation of drawings]

Figure 1 is a diagram showing the modulation frequency dependence of the frequency deviation per unit current of a semiconductor laser with a BH structure, Figure 2 is a waveform diagram during low-speed frequency modulation of a semiconductor laser, and Figure 3 is a diagram of the waveform of a semiconductor laser during low-speed frequency modulation. FIG. 4, a waveform diagram during high-speed frequency modulation, is a diagram showing deterioration when a BHM semiconductor laser is modulated with mB(m+1)13 code. Fig. 5 is a block diagram for explaining the first embodiment of the present invention, Fig. 6 is a waveform diagram of each part of the first embodiment, and Fig. 7 is a diagram for explaining the second embodiment of the present invention. The block diagram of FIG. 8 is a waveform diagram of each part of the second embodiment. In the figure, 1... bias circuit 3... semiconductor laser 4... pulse modulation signal 5... code conversion circuit 7... frequency modulation output 8... auxiliary circuit. Representative Patent Attorney Susumu Uchi' 1-1 Figure IK IOK 100K IM IOM 100M I
G IQ variation and same JL number (Hz) 10 2 Figure ■ 1 Year old 3 Figure fB-! i-Kaichi ↑ 'T31 years old 4 figure + 23456789 years old 5 figure O 6 figure "r7 figure

Claims (1)

[Claims] (1) In a method of frequency modulating the output light of the semiconductor laser by minutely changing the injection current of the semiconductor laser with a pulse modulation signal, ff1Br is added to the pulse modulation signal.
It is characterized by using an LB code, and in particular, the values of m and ru are set so that the mB code satisfies the following conditions: 1≦m≦9.2≦ru≦10, and m<ru. A frequency modulation method for semiconductor lasers.