JPH07193534A - Optical phase locked loop circuit - Google Patents

Abstract

PURPOSE:To generate a repetitive optical clock signal whose frequency is high without being restricted by the operation speed of an optical clock signal generation circuit and to generate the optical clock signal of very high speed CONSTITUTION:An optical coupler 2, an optical modulator 3, a band-pass filter 4, a light-receiving circuit 5, a low frequency oscillator 11, a phase comparator 6, a voltage control oscillator 7, a frequency mixer 8 and a light source generating the optical clock signal are provided. The light source is constituted by an optical pulse generator 9 and an optical pulse multiplex circuit 10. The optical pulse of the output frequency (center frequency f0+DELTAf or f0-DELTAf) the frequency mixer 8 is generated, and the frequency of the optical pulse is multiplied by (n). Thus, a clock signal synchronized with an optical signal modulated at random at very high speed and a frequency-dividing clock signal can be generated by using the optical clock signal.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is used for extracting and generating an optical clock signal for synchronization in an optical repeater, an optical terminal equipment, or other optical signal processing in ultrahigh-speed optical communication.
The present invention relates to an optical phase locked loop circuit that can generate an ultrahigh-speed optical clock signal without being limited by the operating speed of the optical clock signal generation circuit.

[0002]

2. Description of the Related Art In a conventional optical phase locked loop circuit, as shown in FIG. 8, an optical coupler 2 for coupling an optical signal arriving at an input terminal 1 and an optical clock signal, and an output of the optical coupler 2. And a bandpass filter 4 through which an output of the optical modulator 3 passes and a signal corresponding to the optical signal passes.
A light receiving circuit 5 for converting the output of the bandpass filter 4 into an electric signal and extracting a phase component generated by the correlation with the optical clock signal, and a low frequency oscillator 11 for generating an electric signal of low frequency (Δf). A phase comparator 6 that compares the output of the low frequency oscillator 11 with the phase component, a voltage controlled oscillator 7 having a center frequency f ₀ controlled by the output of the phase comparator 6, and the voltage controlled oscillator. The frequency mixer 8 mixes the output of the low frequency oscillator 11 and the output of the low frequency oscillator 11, and the optical pulse generator 9 driven by the output of the frequency mixer 8 to generate the optical clock signal.

The operation of the conventional example thus configured will be described.

The optical signal (repetition frequency f ₀ ) input from the input terminal 1 is multiplexed with the optical clock signal via the optical coupler 2 and input to the optical modulator 3. For simplification of explanation, it is assumed that the optical signal and the optical clock signal are sine waves, and each of them is Ps (t) and Pc (t). Ps (t) = Ps {1 + sin (ωt + φ (t))} (1) Pc (t) = Pc {1 + sin (ω + Δω) t} (2) (Ps and Pc are constants).

Output waveforms Psout and Pcout when these two types of signals are incident on the optical modulator 3 and are subjected to gain modulation are as follows.

Psout = GPs [1 + sin {ωt + φ (t)}] [1 + m (Pc) sin {(ωt + Δω) t + π}] (3) Pcout = GPc {1 + sin (ω + Δω) t} [1 + m (Ps) sin {ωt + φ ( t) + π}] (4) G is the unsaturated gain of the optical modulator 3, Ps is the saturated light intensity of the optical modulator 3, and m (Pc) and m (Ps) are gain modulation by the optical clock signal and the optical signal, respectively. It is degree.

According to this, when the optical clock signal strength reaches its peak, the number of carriers in the optical modulator 3 becomes the smallest.
Therefore, the phase of the gain modulation in the optical modulator 3 is different from the phase of the incident optical clock signal by π, and the term of the phase difference π as shown in the equations (3) and (4) is added. Of the optical signal and the optical clock signal output from the gain-modulated optical modulator 3, only the optical signal is taken out by the bandpass filter 4. Since the gain bandwidth of the optical modulator 3 is about 50 nm, if the wavelengths of the optical signal and the optical clock signal are set apart by several nm, only one light can be extracted by the bandpass filter 4. is there. In fact, a bandpass filter using a dielectric multilayer film has a transmission bandwidth of about 1 nm.

The optical signal taken out by the bandpass filter 4 is inputted to the light receiving circuit 5, but as a light receiving element PIN-
The photocurrent Os (t) when the PD is used is as follows.

[0009]

[Equation 1] e is the electron charge, η is the quantum efficiency of PIN-PD, and hν is the photon energy. The last term in equation (5) is the Δf component caused by the correlation with the optical clock signal,
Therefore, the fluctuation of the phase difference between the optical signal and the optical clock signal is replaced by the fluctuation of the Δf component. The phase comparator 6 performs a phase comparison between the Δf output and the Δf output of the low frequency oscillator 11 and feeds it back to the voltage controlled oscillator 7 to execute the phase locked loop operation.

For specific experimental results, see S. Kawanish
i et al., "10 GHz timing extraction from randomly m
odulated optical pulses using phase-locked loop wi
th traveling-wave laser-diode optical amplifier u
sing optical gain modulation, "Electronics Letters
vol.28, pp.510-511, 1992.

A characteristic of such a configuration is that AW = 0 because phase comparison is performed using the Δf component,
The last term in the equation (5) is an output proportional to φ (t).
Therefore, when the optical phase locked loop circuit operates and φ (t) is 0, the feedback signal to the voltage controlled oscillator 7 is 0.
Therefore, the fluctuation of the power of the input optical signal is not affected. The magnitude of the low frequency modulation frequency Δf is set to such a value that the time variation of φ (t), that is, the pull-in of the optical phase locked loop circuit is set higher than the holding range, and is not affected by the second term of the equation (5). do it.

If Δf is set to a value of about several hundreds of kHz, the electric phase comparator 6 does not need to operate at high speed, so that a phase locked loop operation can be expected.

[0013]

Such an optical phase locked loop circuit has a high speed corresponding to the bit rate of the input optical signal in order to realize the phase lock loop operation for the input optical signal of the high bit rate. Although it is necessary to generate an optical clock signal, looking at the current state of high-speed optical clock signal generation technology, in the semiconductor gain switch method currently mainly used as an optical pulse generation method, the repetition frequency is
The upper limit is about 10 GHz, and it is technically difficult to generate an optical clock signal having a repetition rate higher than this frequency.

The present invention has been made under such a background, and it is possible to generate an optical clock signal at an ultrahigh speed without being limited by the operating speed of the optical clock signal generating circuit. An object is to provide a phase locked loop circuit.

[0015]

According to the present invention, an optical clock signal having a frequency corresponding to the bit rate of an ultrahigh-speed optical clock signal is generated by optically time-division multiplexing optical clock pulses. As a means for optically multiplexing the optical clock signal, the optical clock signal is divided into two and an appropriate time delay is given to one of the divided optical clock signals,
It is characterized in that a circuit for multiplexing again and doubling the repetition frequency of the clock signal or a multistage connection of this circuit is used.

That is, an optical coupler (2) for coupling an optical signal arriving at the input terminal (1) and an optical clock signal, an optical modulator (3) to which the output of this optical coupler is given, and this optical coupler. A bandpass filter (4) through which the output of the modulator passes and a signal corresponding to the optical signal passes through, and the output of this bandpass filter is converted into an electric signal and the phase component generated by the correlation with the optical clock signal is generated. A light receiving circuit (5) for extraction, a low frequency oscillator (11) for generating a low frequency electric signal, a phase comparator (6) for phase comparison between the output of the low frequency oscillator and the phase component, and this phase A voltage controlled oscillator (7) having a center frequency f ₀ controlled by the output of a comparator, a frequency mixer (8) for mixing the output of the voltage controlled oscillator and the output of the low frequency oscillator (11), Driven by the output of the frequency mixer In the optical phase-locked loop circuit that includes a light source for generating a clock signal, the light source, the output frequency of the frequency mixer (8) (f ₀ + Δf or f
Optical pulse generator (9) for generating optical pulse of _0- Δf)
And an optical pulse multiplexing circuit (10) for multiplying the frequency of the optical pulse by n.

The low frequency oscillator (11) is an oscillator of frequency Δf, and is provided with an n-fold frequency multiplier (12) between its output and the phase comparator (7), or its output and the frequency. It is desirable to provide a 1 / n divider with the mixer (8).

[0018]

By using an optical pulse multiplexing circuit composed of an optical pulse generator and an optical delay circuit, an optical clock signal having a high repetition frequency is optically generated, which is equivalent to the speed of a high-speed time division multiplexed optical signal. The optical clock signal synchronized with the high-speed time-division multiplexed optical signal is extracted by generating the optical clock signal for performing the comparison and comparing the phases of the both.

Thus, it is possible to generate a repetitive optical clock signal having a high frequency and to generate an ultrahigh-speed optical clock signal without being restricted by the operating speed of the optical clock generating circuit. Furthermore, by using this optical clock signal, it is possible to generate a clock signal and a divided clock signal that are synchronized with the optical signal that is ultra-high speed random modulated.

[0020]

Embodiments of the present invention will now be described with reference to the drawings. FIG. 1 is a block diagram showing the basic configuration of the embodiment of the present invention.

The embodiment of the present invention comprises an optical coupler 2 for coupling an optical signal arriving at the input terminal 1 and an optical clock signal, and a traveling wave type semiconductor laser amplifier to which the output of the optical coupler 2 is given. An optical modulator 3, a bandpass filter 4 through which the output of the optical modulator 3 passes and a signal corresponding to the optical signal passes, and an output of the bandpass filter 4 is converted into an electric signal to obtain the optical clock signal. A light receiving circuit 5 for extracting a phase component generated by the correlation between the low frequency oscillator 11, a low frequency oscillator 11 for generating a low frequency electric signal, and a phase comparator 6 for phase comparison between the output of the low frequency oscillator 11 and the phase component. , A voltage controlled oscillator 7 having a center frequency f ₀ controlled by the output of the phase comparator 6, a frequency mixer 8 for mixing the output of the voltage controlled oscillator 7 and the output of the low frequency oscillator 11, and this frequency mixing To the output of vessel 8 Ri is driven and a light source for generating the optical clock signal, as a feature of the present invention, the light source, the output frequency of the frequency mixer 8 (f ₀ + Δf or f
_0- Δf) optical pulse generator 9 for generating an optical pulse,
Optical pulse multiplexing circuit 1 for multiplying the frequency of the optical pulse by n
With 0 and. The low-frequency oscillator 11 is an oscillator having a frequency Δf, and an n-fold frequency multiplier 12 is provided between its output and the phase comparator 7.

Output of low frequency oscillator 11 and frequency mixer 8
It is also possible to have a configuration in which a 1 / n frequency divider is provided between and.

Next, the operation of the embodiment of the present invention thus constructed will be described.

The oscillation frequency of the voltage controlled oscillator 107 is f _0.
And The value of f ₀ is set so that the bit rate of the optical signal input from the input terminal 1 is nf ₀ (n is an integer of 1 or more). The output of the voltage controlled oscillator 7 is frequency-shifted by the low frequency oscillator 11 and the frequency mixer 8, and the same operation as in the conventional example is performed up to the point where the optical pulse generator 9 is driven. The feature of the present invention resides in that the frequency of the optical clock signal is multiplexed by the optical pulse multiplexing circuit 10.

FIG. 2 is a diagram showing the configuration of an optical pulse multiplexing circuit using an optical fiber and the waveform of an input / output optical clock signal in the embodiment of the present invention. In the figure, 101 is an input terminal, 102 is an optical fiber coupler, 103 is an optical fiber delay line, 104 is an optical fiber coupler, and 105 is an output terminal. The optical clock signal incident from the input terminal 101 is divided into two by the optical fiber coupler 102. One of the divided optical clock signals is the optical fiber delay line 10
After a delay of T / 2 (T is a time slot of the input optical clock signal = 1 / f ₀ ) is added by 3, the optical fiber coupler 104 multiplexes again the optical clock signal having the double repetition frequency. Is generated. In this case, the repetition frequency of the generated optical clock signal is 2 × (f + Δ
f). When the multiplicity of the optical clock signal is set to be greater than 2 and n is set, n multiplex circuits may be cascaded.
Due to the cascade of k stages, the multiplicity of the optical clock signal becomes 2 ^k . However, in this case, the delay amount of the optical fiber delay line used for the k-th multiplexing circuit from the incident side is T / 2 ^k .

FIG. 3 is a diagram showing the configuration of a three-stage eight-multiplex optical pulse multiplexing circuit using an optical waveguide in the embodiment of the present invention. In the figure, 111 is an input terminal, 112, 115, 11
8, 121 are optical multiplexers, 113, 114, 116, 11
7, 119 and 120 are optical waveguides, and 312 is an output terminal. This circuit has a configuration in which the functions described with reference to FIG. 2 are integrated on a quartz substrate, and the function itself is the same as the configuration in FIG. 2, but the circuit is monolithically integrated, so it is small and has high temperature. It is possible to realize a stable operation that is not affected by the fluctuation of An example of realizing multiplexing of optical pulse signals using this circuit is S. Kawanishi et. Al., "100 Gbit
/ s, 50km, and Non-Repeated Optical Transmission Empl
oying All-Optical Multi / Demultiplexing PLL Timing,
"Electronics Letters, vol.29, pp.1075-1076, 1993.".

The optical clock signal multiplexed by the optical pulse multiplexing circuit is multiplexed with the optical signal via the optical coupler 2 and is incident on the optical modulator 3. At this time, the bit rate nf ₀ of the optical signal, the frequency 2 ^k f ₀ of the optical clock signal after multiplexing and n = 2 ^k to equal, be an optical signal is completely random modulation signal light It is possible to detect the correlation between the signal and the clock signal.

Next, the operation of the optical modulator 3 will be described.
FIGS. 4A and 4B are diagrams showing the relationship between the wavelength λsig of the signal light and the wavelength λclk of the optical clock signal which are incident on the optical modulator in the embodiment of the present invention. When the wavelengths of the optical signal λsig and the optical clock signal λclk are distant from each other as shown in FIG. 9A, the carrier in the optical modulator 3 is modulated by the intensity of the incident optical clock signal. That the carrier is modulated means that the gain of the optical modulator 3 for the signal light that is the other optical input is modulated.
FIG. 6B shows a case where the optical signal λsig and the optical clock signal λclk are close to each other. For details of the principle of this modulator, see S. Kawanishi et al., "10 GHz timing extraction.
from randomly modulated optical pulses using phase
-lockedloop with traveling-wave laser-diode optic
al amplifier using optical gain modulation, "Eletro
nics Letters vol.28, pp.510-511, 1992 for further details.

Note that λx shown in FIG. 4B is λx = 2λsig −λclk, and this wavelength is not directly related to the present invention, so the description thereof will be omitted. Application (Ohm) page 69-72.

As described above, according to the present invention, the optical clock signal is multiplexed by the optical pulse multiplexing circuit 10 so that the correlation with the multiplexed signal (bit rate: nf ₀ ) can be detected by the optical modulator 3. (F ₀ + Δf),
As a result, timing extraction can be performed even for a randomly modulated time division multiplexed signal that does not include the f ₀ component at all. The greatest advantage is that the frequency of the optical clock signal is nf
_It does not require a high-speed laser that directly generates an optical clock signal of _zero .

This operation can be understood in the same manner as the above-mentioned conventional example device, and in the equation (5), Δω is read as nΔω,

[0032]

[Equation 2] Becomes

FIG. 5 is a diagram showing the configuration of an experimental system according to the embodiment of the present invention. The optical clock signal from the oscillator 201 is a mode-locked laser (hereinafter referred to as EDF laser) 202
The optical clock signal of 6.3 GHz at a wavelength of 1.552 μm is transmitted from the EDF amplifier 205 at 6.3 Gbit / s, and the optical clock signal of 1.535 μm from the gain modulation laser (hereinafter referred to as DFB laser) 203 is normal. It is linearly compressed through a compression fiber 204 that performs dispersion, and is sent out from the EDF amplifier 206 as a signal of 6.3 GHz + Δf.

The two optical clock signals are supplied by the waveform multiplex optical coupler 207, and the multiplex circuit 20 is constituted by the interferometer type planar lightwave circuit having the three-stage structure of quartz system.
8 multiplexed, 50 Gbit / s optical signal and 50 GH
It is transmitted as an optical clock signal of z. Multiplex circuit 208
Then, the time delay given to each stage is predetermined, and in the case of this example, 1.5T (238 ps), 0.7
It is 5T (119 ps) and 0.375 T (59.5 ps). Here, T is a time width of 6.3 GHz, and in order to avoid uncertainty of the polarity of the feedback signal, the repetition frequency of the original optical clock signal is from 6.3 GHz to Δ.
It is shifted by f. Therefore, the multiplexed 5
The frequency fclk of the optical clock signal of 0 GHz is calculated by the following equation.

Fclk = 50 GHz + 8Δf (5) This means that the correlation between the optical signal of 50 Gbit / s and the multiplexed optical clock signal is 8Δf. Therefore, the 8Δf component that is the correlation signal and the original Δf
The voltage controlled oscillator 7 outputs the phase comparison output with the 8th harmonic of the signal.
If it is fed back to, the optical phase synchronization operation is realized.

Since only one optical phase-locked loop circuit can be used in this experiment, the lights of the optical signal and the optical clock signal are combined by the wavelength-multiplexing optical coupler 207 and combined by one multiplexing circuit 208. The optical signal is multiplexed. It should be noted here that since the wavelengths of both are separated, the multiplexing of the optical signal and the multiplexing of the optical clock signal are performed independently.
When the output of the multiplex circuit 208 was input to an optical modulator having a multiple quantum well structure, the peak gain was 1.53 μm and 8 dB.

The signal multiplexed in this way is the EDF.
Amplifier 209 and laser amplifier (after being amplified by the traveling wave semiconductor laser 210, only the optical signal of 1.552 μm is taken out by the bandpass filter 211, and the photoelectric converter (AP
D) is input to 212. The output including the 8Δf component (351 kHz) which is the correlation signal from the photoelectric converter 212 is input to the phase comparator 213. The input signal is compared with the 8Δf signal obtained by electrically multiplying the Δf oscillator by eight, and the output thereof is fed back to the voltage controlled oscillator 214.

FIG. 6A is a diagram showing the waveform of an optical signal of 50 Gbit / s measured at the incident end of the optical modulator 3, FIG. 6B.
Shows a power spectrum of an optical signal and an optical clock signal, FIG. 7A shows a power spectrum of a cross-correlation signal at an APD output, and FIG. 7B shows an output waveform of an optical modulator. Is. According to this, the pulse width of the optical signal is 6.8.
Although it was ps, it becomes 4.6 ps when calibrated considering the resolution of the streak camera used for observation of 5 ps. The powers of the optical signal and the optical clock signal incident on the optical modulator 3 were both +7 dBm, and the signal-to-noise ratio (S / N) of the cross-correlation signal was 25 dB. Here, the output 6.3 GHz from the voltage controlled oscillator 214 was used as the trigger signal of the oscilloscope 215. The peak gain of the optical modulator 3 was 7 dB at a wavelength of 1.552 μm.

By optimizing the optical modulator 3, at least 3
The gain of dB can be expected to be improved, and the S / N of the cross-correlation signal can be expected to be improved by 6 dB or more, and the purity (jitter) of the optical clock signal is mainly S of the cross-correlation signal.
/ N, and other factors such as the response speed of the optical modulator 3 can be ignored.
It can also be expected to be used as a 0 GHz phase comparator.

[0040]

As described above, according to the present invention, it is possible to generate a repetitive optical clock signal having a high frequency by optically multiplexing the optical clock signal, and to use the optical clock signal generating circuit. An optical clock signal that can generate an ultrahigh-speed optical clock signal without being restricted by the operating speed, and by using the optical clock signal, an optical clock signal synchronized with the ultrahigh-speed randomly modulated optical signal and its divided clock signal There is an effect that can be generated.

[Brief description of drawings]

FIG. 1 is a block diagram showing a basic configuration of an embodiment of the present invention.

FIG. 2 is a diagram showing a configuration of an optical pulse multiplexing circuit using an optical fiber and a waveform of an input / output optical clock signal in the embodiment of the present invention.

FIG. 3 is a three-stage 8 using an optical waveguide according to an embodiment of the present invention.
The figure which shows the structure of a multiplex optical pulse multiplexing circuit.

4A and 4B are diagrams showing the relationship between the wavelength λsig of the signal light incident on the optical modulator and the wavelength λclk of the optical clock signal in the embodiment of the present invention.

FIG. 5 is a block diagram showing the configuration of an experimental system according to an example of the present invention.

FIG. 6A is a diagram showing a measured waveform of a 50 Gbit / s optical signal, and FIG. 6B is a diagram showing spectra of the optical signal and the optical signal.

7A is an oscillographic waveform diagram showing a power spectrum of a cross-correlation signal at an APD output, and FIG. 7B is an oscillographic waveform diagram showing an output waveform of an optical modulator.

FIG. 8 is a block diagram showing a configuration of a conventional example.

[Explanation of symbols]

1, 101, 111 Input terminal 2 Optical coupler 3 Optical modulator 4, 211 Bandpass filter 5 Light receiving circuit 6, 213 Phase comparator 7, 214 Voltage controlled oscillator 8 Frequency mixer 9 Optical pulse generator 10 Optical pulse multiplex circuit 11 Low Frequency Oscillator 12 Frequency Multiplier 102, 104 Optical Fiber Coupler 103 Optical Fiber Delay Line 105, 122 Output Terminal 112, 115, 118, 121 Optical Multiplexer 113, 114, 116, 117, 119, 120 Optical Waveguide 201 Oscillator 202 EDF laser 203 DFB laser 204 Compressed fiber 205, 206, 209 EDF amplifier 207 WDM optical coupler 208 Multiple circuit 210 Laser amplifier 212 Photoelectric converter 215 Oscilloscope

Continuation of the front page (51) Int.Cl. ⁶ Identification number Office reference number FI Technical indication location H04L 7/02 B (72) Inventor Tsutomu Kito 1-1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nippon Telegraph and Telephone Stock In the company

Claims (3)

[Claims] 1. An optical coupler (2) for coupling an optical signal arriving at an input terminal (1) and an optical clock signal, an optical modulator (3) to which an output of the optical coupler is given, and an optical modulator (3) A bandpass filter (4) through which the output of the modulator passes and a signal corresponding to the optical signal passes through, and the output of this bandpass filter is converted into an electric signal and the phase component generated by the correlation with the optical clock signal is generated. A light receiving circuit (5) for extracting, a low frequency oscillator (11) for generating a low frequency electric signal, and a phase comparator (6) for phase comparison between the output of the low frequency oscillator and the phase component.
And a voltage controlled oscillator (7) having a center frequency f ₀ controlled by the output of this phase comparator, and a frequency mixer (8) for mixing the output of this voltage controlled oscillator and the output of the low frequency oscillator (11). ), And an optical phase-locked loop circuit driven by the output of the frequency mixer to generate the optical clock signal, the light source comprising an output frequency (f) of the frequency mixer (8).
An optical phase comprising an optical pulse generator (9) for generating an optical pulse of ₀ + Δf or f ₀ -Δf) and an optical pulse multiplexing circuit (10) for multiplying the frequency of the optical pulse by n. Synchronous loop circuit. 2. The low frequency oscillator (11) has a frequency Δf.
2. The optical phase-locked loop circuit according to claim 1, further comprising an n-fold frequency multiplier (12) between the output of the oscillator and the phase comparator (7). 3. The low frequency oscillator (11) has a frequency nΔ
f is an oscillator, and its output and the frequency mixer (8)
The optical phase-locked loop circuit according to claim 1, further comprising a 1 / n frequency divider between and.