JPH0526736A - Modulation index measurement system in light direct modulation psk system and method for controlling semiconductor laser using it - Google Patents

Abstract

PURPOSE:To enable a modulation index to be measured easily. CONSTITUTION:An intermediate frequency signal obtained by detecting what is obtained by superposing a signal light modulated by the PSK system with a local oscillation light is doubled by a frequency doubler 5 and is fed to a frequency analyzer 6, thus obtaining a modulation index based on a frequency difference of large two frequency components which appear at the frequency analyzer 6.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a modulation index measuring system in an optical direct modulation PSK system and a semiconductor laser control method using the system.

As an optical fiber transmission system currently put into practical use, an intensity modulation / direct detection (IM / DD) system is generally used in which intensity-modulated light is directly received by a photodiode and converted into an electric signal. . On the other hand, in recent years, coherent optical fiber transmission systems have been actively researched due to demands such as an increase in transmission capacity and an increase in transmission distance.

According to this system, the coherent light from the semiconductor laser is used as a carrier to modulate the frequency, phase, etc., and for example, the receiving light and the local light are mixed at the receiving side to perform heterodyne detection or homodyne detection. Since this is done, it is possible to greatly improve the reception sensitivity as compared with the IM / DD system.

Further, after performing optical detection, that is, after converting an optical signal into an electric signal, it is possible to relatively easily perform highly accurate frequency selection, which enables high-density frequency division multiplexing, The transmission capacity of a single optical transmission line can be dramatically increased.

[0005]

2. Description of the Related Art Conventionally, information to be transmitted is placed on a wave parameter of light from a semiconductor laser and suitable for high-speed transmission, as a DPSK (differential phase shift keying) method or a CPFSK (continuous phase). A frequency shift keying method is known.

In order to deal with the drawbacks of the DPSK system and the CPFSK system, we have previously proposed an optical direct modulation PSK system (sometimes referred to as "DM-PSK system" in the present specification). .

In this system in the case where the binary code format is adopted, the injection current to be given to the semiconductor laser which outputs the light of the frequency corresponding to the injection current is supplied to one time slot of the binary-coded input signal. The signal light is transmitted by changing the predetermined value for a shorter period of time so that the integrated value of the frequency fluctuated according to the change of the injection current becomes π or −π as the phase amount.

This system does not require a differential encoding circuit and an external modulator and is suitable for direct modulation of a semiconductor laser. It is less susceptible to chromatic dispersion than the CPFSK system, and can reproduce a carrier signal. It has the feature.

[0009]

In the DM-PSK system, the modulation index is an important parameter as a ratio between the frequency shift amount and the bit rate. If the modulation index is set to a value different from the required value, the receiving sensitivity is deteriorated or the wavelength is likely to be affected by chromatic dispersion, and an optimum optical communication system cannot be constructed. Therefore,
In order to construct an optimal optical communication system, facilitation of measurement of modulation index is a technical issue.

It is an object of the present invention to provide a modulation index measuring system which can easily obtain a modulation index in the DM-PSK system. It is also an object of the present invention to provide a semiconductor laser control method using this measurement system.

[0011]

FIG. 1 is a block diagram showing the first constitution of the modulation index measuring system of the present invention. 1
Is a binary optical direct modulation PSK system (binary DM-PSK system)
Is an optical transmission device to which the optical transmission device 1 is applied.
The injection current given to the semiconductor laser that outputs light having a frequency corresponding to the injection current is set to 1 of the binary-coded input signal.
Change by a predetermined time shorter than the time slot,
The signal light is sent out so that the integrated value of the frequency fluctuated according to the change of the injection current becomes π or −π as the phase amount.

This modulation index measuring system includes an optical local oscillation circuit 2 for outputting a local oscillation light, an optical superposition circuit 3 for superposing the local oscillation light on the signal light, a signal light from the optical superposition circuit 3 and An optical detection circuit 4 for optoelectrically converting local oscillation light to generate an intermediate frequency signal, a frequency doubler 5 for doubling the frequency of the intermediate frequency signal, and a frequency component of the signal from the frequency doubler 5. The frequency index 6 is provided, and the modulation index is obtained based on the frequency difference between the two large frequency components appearing in the frequency analyzer 6.

FIG. 2 is a block diagram showing a second configuration of the modulation index measuring system of the present invention. Reference numeral 11 denotes an optical transmission device to which an n-valued optical direct modulation PSK system (n-valued DM-PSK system) is applied. The optical transmission device 11 supplies the semiconductor laser which outputs light having a frequency corresponding to an injection current. The injection current is changed for a predetermined time shorter than one time slot of an n-value encoded (n = 2 ^m (m is a natural number of 2 or more)) input signal, and the frequency fluctuates according to the change of the injection current. The signal light is transmitted so that the integrated value of 2 becomes 2πk / n or −2πk / n (k = 1, 2, ..., (n−1)) as the phase amount.

This modulation index measuring system includes an optical local oscillation circuit 2 for outputting a local oscillation light, an optical superposition circuit 3 for superposing the local oscillation light on the signal light, a signal light from the optical superposition circuit 3, and An optical detection circuit 4 for optoelectrically converting the locally oscillated light to generate an intermediate frequency signal, a frequency converter 12 for multiplying the frequency of the intermediate frequency signal by n, and the frequency converter 1.
2 and a frequency analyzer 6 that analyzes the frequency components of the signal from 2, and the modulation index is obtained based on the mutual frequency difference of the large n frequency components appearing in the frequency analyzer 6.

FIG. 3 is a block diagram showing a third configuration of the modulation index measuring system of the present invention. In the third configuration, the same optical transmission device 1 as in the first configuration is used.

This modulation index measuring system includes an optical frequency discriminator 21 for outputting the signal light with an intensity corresponding to its frequency, and an optical detection circuit for optoelectrically converting the signal light from the optical frequency discriminator 21. 4, a frequency doubler 5 that doubles the frequency of the signal from the photodetector circuit 4, and a frequency analyzer 6 that analyzes the frequency components of the signal from the frequency doubler 5.
And the modulation index is obtained based on the frequency difference between two large frequency components appearing in the frequency analyzer 6.

FIG. 4 is a block diagram showing a fourth configuration of the modulation index measuring system of the present invention. In the fourth configuration, the same optical transmission device 11 as in the first configuration is used.

This modulation index measuring system comprises an optical frequency discriminator 21 for outputting the signal light with an intensity corresponding to its frequency, and an optical detection circuit for optoelectrically converting the signal light from the optical frequency discriminator 21. 4, a frequency converter 12 that multiplies the frequency of the signal from the photodetector circuit n by n, and a frequency analyzer 6 that analyzes the frequency components of the signal from the frequency converter 12. The modulation index is obtained based on the mutual frequency difference between the n large frequency components shown in FIG.

According to the semiconductor laser control method of the present invention, in the modulation index measuring system according to any one of the first to fourth configurations of the present invention, the half width of the spectrum waveform of the frequency component appearing in the frequency analyzer 6 is the most. To be smaller
The frequency shift amount or the predetermined time in the binary or n-valued DM-PSK system is controlled.

[0020]

The first or third of the modulation index measuring system of the present invention
With the configuration, the modulation index in the binary DM-PSK system can be easily measured based on the frequency difference between the two large frequency components appearing in the frequency analyzer 6.

According to the second or fourth configuration of the modulation index measuring system of the present invention, a large n appearing in the frequency analyzer 6 is displayed.
N-value DM based on the mutual frequency difference of the frequency components
-It becomes possible to easily measure the modulation index in the PSK method.

When the semiconductor laser control method of the present invention is implemented by using the first or third configuration of the modulation index measurement system of the present invention, the half width of the spectrum waveform of the frequency component appearing in the frequency analyzer 6 is shown. Since the frequency shift amount in the binary DM-PSK system or the above predetermined time is controlled so that is minimized, the phase shift amount of the signal light becomes exactly π, and the reception sensitivity is improved. It is possible to improve the proof strength of wavelength dispersion.

When the semiconductor laser control method of the present invention is implemented by using the second or fourth configuration of the modulation index measuring system of the present invention, the half width of the spectrum waveform of the frequency component appearing in the frequency analyzer 6 is shown. Since the frequency shift amount in the n-valued DM-PSK system or the above-mentioned predetermined time is controlled so as to be the smallest, the phase shift amount of the signal light is accurately set to 2πk / n (k = 1, 2). , ..., (n-1))
It becomes possible to improve the receiving sensitivity and the chromatic dispersion tolerance.

[0024]

EXAMPLES Examples of the present invention will be described below. FIG. 5 is a block diagram of a transmission / reception system to which the binary DM-PSK scheme is applied.

In the transmitter 1, 31 is a semiconductor laser which outputs light having a frequency corresponding to the injection current, 32 is a bias current circuit which gives a bias current to the semiconductor laser 31,
33 is one time slot T of the binary-coded input signal
The drive circuit superimposes a modulated current pulse having a shorter pulse width on the bias current.

The amplitude and pulse width of the modulation current pulse are controlled so that the integrated value of the frequency of the signal light varied by the modulation current pulse becomes π or −π as the phase amount. D
The bias current composed of the C component is supplied to the semiconductor laser 31 via the inductor 34, and the high-speed modulated current pulse is supplied to the semiconductor laser 31 via the capacitor 35.

The drive circuit 33 is constructed, for example, by inputting an input signal in the NRZ code format and a clock signal into an OR / NOR circuit and obtaining an RZ signal as its output. The signal light is transmitted to the receiving side via the optical fiber 36. This signal light is superposed on the local oscillation light from the local oscillation semiconductor laser 37 in the optical coupler 38. The signal light and the local light are converted into an intermediate frequency signal by the optical detection circuit 39 and demodulated by the demodulation circuit 40.

The demodulation circuit 40 branches the input intermediate frequency signal and applies a delay of one time slot T to one of them by a delay circuit 41, and outputs the delayed signal and the undelayed signal. It is configured to be mixed by the mixer 42.

FIGS. 6A and 6B are explanatory diagrams of the operation principle of the binary DM-PSK system. FIG. 6A is a modulation driving waveform, FIG. 6B is an intermediate frequency signal (IF signal) waveform, and FIG. 6C is a demodulation waveform. Respectively. The modulation code of each waveform is "0101".
It is when it is.

FIG. 7 is a graph showing changes over time in the frequency shift ΔF and the phase change Δθ. The modulation code in this case is "01101". In the binary DM-PSK system, the oscillation frequency of the semiconductor laser is shifted by ΔF for a predetermined time τ in one time slot T, and thereafter the oscillation frequency is returned to the original frequency. And τ,
ΔF is set so that the phase shift becomes π or −π after the time τ has elapsed. That is, the setting is made so that the relationship between τ and T satisfies the following equation.

Τ = T / 2m where m is a modulation index, and this modulation index is defined as follows using the frequency deviation ΔF and the bit rate B.

M = ΔF / B FIG. 8 is a diagram showing a setting example of the drive waveform according to the modulation index. If m = 1,
τ = T / 2 and ΔF = B. When m = 2, τ = T / 4 and ΔF = 2B.

Since the modulation index m is closely related to other various parameters as described above, it is necessary to accurately grasp the modulation index m in order to construct a system that functions reliably.

According to the DM-PSK system, an external modulator and a differential encoder are unnecessary, so that the structure of the transmission system can be simplified. Further, since the FM modulation characteristic of the semiconductor laser is generally 10 GHz or higher, high speed transmission becomes possible. Furthermore, compared to the CPFSK system, it is less susceptible to chromatic dispersion of the optical fiber, so long-distance transmission is possible.

FIG. 9 is a block diagram showing an embodiment of the first constitution of the modulation index measuring system of the present invention. In this embodiment, the intermediate frequency of 2 output from the frequency doubler 5 is used.
The optical local oscillator circuit 2 is provided with the automatic frequency control circuit 51 so that the signal corresponding to the doubled frequency is maintained at a constant frequency.
Control the oscillation frequency of. The control target in this case is, for example, the bias current or the temperature of the semiconductor laser used in the optical local oscillation circuit 2.

By performing such automatic frequency control, the spectrum of the signal input to the frequency analyzer 6 can be kept substantially constant, and accurate modulation index measurement becomes possible.

The measurement principle in this embodiment will be described below. By using the frequency doubler, the phase modulation component based on 0, π modulation can be canceled, and the required frequency component can be extracted by using an appropriate bandpass filter.

Now, the intermediate frequency signal IF ₁ (t) is set as IF ₁ (t) = cos (2πf _IF t + θ (a, t)) (0 ≦ t ≦ 1 / B) (1). However, a is a transmission code (0 or 1), f _IF is a frequency corresponding to the code a = 0, and θ (a, t) is defined by the following equation.

┌─0 ((a) a = 0) θ (a, t) = ┤2πmBt ((b) a = 1 and 0 ≦ t ≦ 1 / (2mB)) (2) └─π (( ) a = 1 and 1 / (2 mB) ≤t≤1 / B) where m is the modulation index and B is the transmission rate (bit rate)
Is.

(A) and (c) show the states of constant phases 0 and π, respectively, and (b) show the states where the phase shifts from 0 to π. Output signal IF ₂ (t) of frequency doubler 5
Becomes as follows according to the value of θ (a, t).

┌─cos 2π ・ 2f _IF t ((a) or (c)) IF ₂ (t) = │ (3) └─cos 2π (2f _IF + 2mB) t ((b)) Therefore, the output of the frequency doubler 5 The signal IF ₂ (t) has two large frequency components 2f _IF and 2f _IF + 2mB, and the frequency difference between them is 2mB.

FIG. 10 is a diagram showing an example of actual measurement of the spectral densities of the intermediate frequency signal and the output signal of the frequency doubler. The dotted line is for the intermediate frequency signal and its center frequency is 4.5 GHz. Here, the reason why the center frequency of the intermediate frequency signal and the peak frequency do not match is that the modulation is performed based on the asymmetric NRZ signal.

The solid line shows the output signal of the frequency doubler 5. There is a peak A of one frequency component at a frequency of 9 GHz and a peak B of another frequency component at a frequency of 14 GHz.

Based on equation (3), peak A and peak B
That is, the frequency difference of 1 corresponds to 2 mB. Since the frequencies giving the peaks A and B can be obtained as information from the frequency analyzer 6, the measured value and the bit rate B
The modulation index m is calculated from

By the way, when the phase shift amount deviates from π,
A phase difference occurs between (a) and (c) in the equation (3), and the frequency component giving the above-mentioned peak contains many complex frequency components, and the half-width of the spectrum of the frequency component giving the peak is Increase. Therefore, by controlling ΔF or τ in the optical transmitter 1 so that this half width becomes the smallest, accurate 0-π modulation becomes possible,
It becomes possible to maintain high reception sensitivity.

FIG. 11 shows an optical transmitter 1 which can be used to implement the second configuration of the modulation index measuring system of the present invention.
It is a block diagram of 1. In this example, n = 4, so four-level modulation is performed.

2-channel input signals m ₁ (t) and m ₂ (t)
Among them, the former amplitude is doubled by the amplifier 61, and the input signal m ₁ (t) whose amplitude is doubled and the other input signal m ₂ (t) whose amplitude is not doubled are added. After being added by the device 62, this is input to the drive circuit 63.

According to this configuration, of the four-valued states represented by the input signal, the first state is not frequency-shifted, and the second to fourth states are relative to the first state. The phase shift is 2πk / 4 (k = 1, 2,
A modulated current pulse is generated so that 3).

That is, when the phase state corresponding to one of the four-valued signals is 0, the other three phase states are π / 2, π and 3π / 2, respectively. FIG. 12 is a block diagram showing an embodiment of the second configuration of the modulation index measuring system of the present invention. As the optical transmitter 11, the one shown in FIG. 11 is used. In this case, since it is four-value modulation,
Frequency converter 12 for multiplying the frequency of the intermediate frequency signal by n
Is configured by connecting two frequency doublers 71 and 72 in series as shown in the figure.

In the present embodiment, four large frequency components appear in the frequency analyzer 6, so that the modulation index can be easily calculated based on the mutual frequency difference. Since the measurement principle in this case is the same as that in the first embodiment, the description thereof will be omitted.

The third embodiment of the modulation index measuring system of the present invention.
The fourth and fourth configurations can be implemented according to the first or second configuration. In this case, the optical frequency discriminator 2
1 is a Fabry-Perot interferometer or other optical interferometer,
A fiber fusion type multiplexer / demultiplexer or the like can be used.

[0052]

As described above, according to the present invention,
It is possible to provide a modulation index measuring system capable of easily measuring the modulation index.

Further, by controlling the semiconductor laser using this system, it becomes possible to optimally set the phase shift amount in the optical direct modulation PSK system, improving the receiving sensitivity and the chromatic dispersion tolerance. Has the effect that it becomes possible.

[Brief description of drawings]

FIG. 1 is a block diagram showing a first configuration of a modulation index measurement system of the present invention.

FIG. 2 is a block diagram showing a second configuration of the modulation index measurement system of the present invention.

FIG. 3 is a block diagram showing a third configuration of the modulation index measurement system of the present invention.

FIG. 4 is a block diagram showing a fourth configuration of the modulation index measurement system of the present invention.

FIG. 5 is a block diagram of a transmission / reception system to which a binary DM-PSK scheme is applied.

FIG. 6 is an explanatory diagram of an operation principle of a binary DM-PSK system.

FIG. 7 is a graph showing changes over time in frequency shift and phase change in the binary DM-PSK system.

FIG. 8 is a diagram showing an example of setting a drive waveform according to a modulation index.

FIG. 9 is a block diagram showing an example of a first configuration of the modulation index measurement system of the present invention.

FIG. 10 is a diagram showing the spectral densities of the intermediate frequency signal and the output signal of the frequency doubler.

FIG. 11 is a block diagram of an optical transmitter used for implementing the second configuration of the modulation index measurement system of the present invention.

FIG. 12 is a block diagram showing an example of a second configuration of the modulation index measurement system of the present invention.

[Explanation of symbols]

1,11 Optical transmitter 2 Optical local oscillation circuit 3 Optical superposition circuit 4 Optical detection circuit 5 frequency doubler 6 frequency analyzer 12 Frequency converter 21 Optical frequency discriminator

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification number Reference number within the agency FI Technical indication location H04B 10/08 H04L 27/18 A 9297-5K (72) Inventor Hideyuki Miyata Nakahara-ku, Kawasaki-shi, Kanagawa 1015 Kamiodanaka, Fujitsu Limited

Claims (5)

[Claims] 1. The injection current applied to a semiconductor laser (31) that outputs light having a frequency according to the injection current is changed by a predetermined time shorter than one time slot of a binary-coded input signal. A modulation index measurement system in a binary optical direct modulation PSK system that sends out signal light such that an integrated value of a frequency that fluctuates according to a change in the injection current becomes π or −π as a phase amount, and is a local oscillation light. Optical local oscillation circuit (2) that outputs the optical signal and an optical superposition circuit (3) that superimposes the local oscillation light on the signal light
And an optical detection circuit (4) for optoelectrically converting the signal light and the local oscillation light from the optical superposition circuit (3) to generate an intermediate frequency signal.
And a frequency doubler for doubling the frequency of the intermediate frequency signal (5)
And a frequency analyzer (6) for analyzing the frequency components of the signal from the frequency doubler (5), and the modulation index is calculated based on the frequency difference between the two large frequency components appearing in the frequency analyzer (6). Modulation index measurement system characterized by seeking. 2. An input signal obtained by performing n-value encoding (n = 2 ^m (m is a natural number of 2 or more)) on the injection current given to the semiconductor laser (31) that outputs light having a frequency according to the injection current. Change for a predetermined time shorter than one time slot, and the integrated value of the frequency changed according to the change of the injection current is 2πk / n or −2πk / n (k =
1, 2, ..., (N-1)), which is a modulation index measurement system in an n-valued optical direct modulation PSK system that outputs signal light, and an optical local oscillation circuit (2) that outputs local oscillation light ), And an optical superimposing circuit (3) that superimposes the local oscillation light on the signal light.
And an optical detection circuit (4) for optoelectrically converting the signal light and the local oscillation light from the optical superposition circuit (3) to generate an intermediate frequency signal.
And a frequency converter (12) for multiplying the frequency of the intermediate frequency signal by n times
And a frequency analyzer (6) for analyzing the frequency component of the signal from the frequency converter (12), based on the mutual frequency difference of the large n frequency components appearing in the frequency analyzer (6). A modulation index measuring system characterized by obtaining a modulation index by 3. The injection current given to a semiconductor laser (31) that outputs light having a frequency according to the injection current is changed by a predetermined time shorter than one time slot of a binary encoded input signal. A modulation index measuring system in a binary optical direct modulation PSK system that sends out signal light such that an integrated value of a frequency that fluctuates according to a change in the injection current becomes π or −π as a phase amount. And an optical frequency discriminator (21) for outputting the signal light from the optical frequency discriminator (21) for opto-electric conversion of the signal light from the optical frequency discriminator (21), and the optical detection circuit A frequency doubler (5) for doubling the frequency of the signal from (4) and a frequency analyzer (6) for analyzing the frequency components of the signal from the frequency doubler (5). 6) based on the frequency difference between the two large frequency components Modulation index measurement system and obtains the modulation index Te. 4. An input signal obtained by performing n-value encoding (n = 2 ^m (m is a natural number of 2 or more)) on the injection current given to the semiconductor laser (31) that outputs light having a frequency according to the injection current. Change for a predetermined time shorter than one time slot, and the integrated value of the frequency changed according to the change of the injection current is 2πk / n or −2πk / n (k =
1, 2, ..., (N-1)), which is a modulation index measurement system in an n-valued optical direct modulation PSK system for transmitting signal light so that the signal light has an intensity according to its frequency. The optical frequency discriminator (21) for outputting, the optical detection circuit (4) for optoelectrically converting the signal light from the optical frequency discriminator (21), and the frequency of the signal from the optical detection circuit (4) A frequency converter (12) for multiplying n times and a frequency analyzer (6) for analyzing frequency components of a signal from the frequency converter (12) are provided, and n large number appearing in the frequency analyzer (6). A modulation index measuring system, characterized in that the modulation index is obtained based on the mutual frequency difference of the frequency components of. 5. The binary or n-valued optical direct light in the system according to claim 1, wherein the half-value width of the spectrum waveform of the frequency component appearing in the frequency analyzer (6) is minimized. A method of controlling a semiconductor laser, comprising controlling a frequency shift amount or a predetermined time in the modulation PSK method.