JP2001094514A - Method and device for optical transmission - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make a device small-sized by using two optical heterodyne detection having a stable frequency interval without using a high frequency oscillator. SOLUTION: Light emitted by LDs 1a and 1b is multiplexed and demultiplexed by a directional coupler 3a; and one of the output light becomes the output of an optical transmitter and the other one is inputted to a directional coupler 3b. Only one of the two demultiplexed output light passes through a delay line 7 and is outputted to optical detectors 4a and 4b and a phase comparator 5 makes phase comparison and outputs phase difference information. The output signal of the phase comparator 5 is inputted to a feedback circuit 6 to extract only phase difference information of low frequency and the information is sent as feedback information to a drive circuit 2, which corrects oscillation wavelength under the control of temperature or/and the drive current so that the frequency interval between the LDs 1a and 1b becomes equal to a desirable high angular frequency.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an optical transmission method and an optical transmitter used for optical subcarrier transmission for transmitting a high-frequency signal by heterodyne detection of two lights having two wavelengths close to each other.

[0002]

2. Description of the Related Art Since optical subcarrier transmission can transmit a high-frequency signal to a distant place via an optical fiber, application to wireless communication in a high-frequency band has been studied. Therefore, optical transmission used for optical subcarrier transmission has been studied. Research on the machine has been actively conducted.

As a candidate for an optical signal source that generates a high-frequency signal as an optical subcarrier signal, a laser diode (L
D) and an external optical modulator (for example, WE
Stephens, and TRJoseph, "System characteristics of
direct modulated and externally modulated RF fibe
r-optic links, "J. Lightwave Technol., vol.5, pp.380-
387, 1987), using a mode-locked laser (for example, K. Sato, K. Wakita, I. Kotaka, Y. Kondo, M. Yamamoto an
d A.takada, "Monolithic strained-InGaAsP multiple-q
uantum-well lasers with integrated electroabsorpti
on modulators for active mode locking, "Appl.Phys.
Lett., Vol. 65, pp, 1-3, 1994), however, when a single mode optical fiber is used, there is a problem that high-frequency components are attenuated during transmission due to the influence of chromatic dispersion.

On the other hand, those using two-light heterodyne detection (for example, N. Timofeev, S. Bennett, R. Griffin, P. Bayve)
l, AJSeeds, R.Wyatt, R.Kashyap and M.Robertson, "Hi
gh spectral purity millimetre-wave modulated optic
al signal generation usingfiber grating lasers ", El
ectron. Lett., vol. 34, pp. 668-669, 1998) has the advantage that high-frequency signals can be transmitted over long distances without being affected by chromatic dispersion. However, an optical transmitter that uses a high-frequency signal using heterodyne detection of two lights as an optical subcarrier signal requires strict control of the wavelength interval, and thus generally includes a feedback circuit.

FIG. 4 is a block diagram showing an example of a conventional optical transmitter. In FIG. 4, reference numerals 31a and 31b denote laser diodes (LDs), 32 denotes a driving circuit, 33 denotes a directional coupler, and 34 denotes a directional coupler.
Is a photodetector, 35 is a phase comparator, 36 is a feedback circuit, 37
Is an oscillator. The bold line in the figure indicates the flow of the optical signal,
Thin lines indicate the flow of electric signals.

Each of the LDs 31a and 31b oscillates in a single mode, and the difference between the two oscillation wavelengths corresponds to the wavelength interval of the high-frequency signal. The drive current and temperature of the LDs 31 a and 31 b are controlled by the drive circuit 32. Directional coupler 33
Has two inputs and two outputs. The photodetector 34 uses a photodiode operating in a high frequency band, and the phase comparator 35 uses a double balance mixer (DBM). The oscillator 37 is formed of a three-dimensional circuit using a Gunn diode, an imput diode, or the like, and outputs a high-frequency signal having a predetermined frequency.

Light generated by the LDs 31a and 31b is multiplexed / demultiplexed by a directional coupler 33, one of which is output light of an optical transmitter, and the other is input to a photodetector 34. A high-frequency signal detected by the photodetector 34 is guided to one high-frequency input terminal of the phase comparator 35, and a signal from the oscillator 37 is guided to the other high-frequency input terminal. Compare the phases of both signals. The output signal of the phase comparator 35 is input to the feedback circuit 36, and the feedback information extracted by the feedback circuit 36 is sent to the drive circuit 32. The drive circuit 32 corrects the oscillation wavelength by controlling the temperature, the drive current, or both so that the frequency interval between the LDs 31a and 31b becomes equal to a predetermined high-frequency frequency. With such a loop configuration, optical subcarrier signals with stable frequency intervals are transmitted from the output of the optical transmitter of the directional coupler 33.

[0008]

As described above, the conventional optical transmitter uses an oscillator. However, since this oscillator is large, the size of the entire optical transmitter becomes large. was there.

The present invention has been made in view of such a problem, and an object of the present invention is to use a heterodyne detection of two light beams having a stable frequency interval without using a high-frequency oscillator to reduce the size. It is an object of the present invention to provide an optical transmission method and an optical transmitter so as to realize an optical transmission method.

[0010]

In order to achieve the above object, an optical transmission method according to the present invention combines and demultiplexes the output lights of a pair of laser diodes and outputs the light output by two-light heterodyne detection. In an optical transmission method for comparing a phase of a subcarrier signal with a reference high-frequency signal and controlling an oscillation wavelength difference of the laser diode to a predetermined frequency interval of the optical subcarrier signal, the reference high-frequency signal is one of The signal is a delayed signal that is delayed after the part is taken out.

With such a configuration, by providing a phase difference to each optical subcarrier high-frequency signal input to the phase comparator by the delay means, phase difference information is extracted from the output of the phase comparator, and feedback for wavelength control is performed. Since it can be used for information, a high-frequency oscillator is not required, and an optical transmission method using heterodyne detection of two lights having a stable frequency interval with a small configuration can be realized.

Further, the frequency interval of the delay signal can be controlled more accurately by selecting an arbitrary natural multiple of 波長 wavelength of the optical subcarrier frequency.

An optical transmitter according to the present invention is an optical transmitter using heterodyne detection of two lights used in optical subcarrier transmission, wherein the optical transmitter has a pair of optical wavelengths separated by a frequency interval of the optical subcarrier signal. A laser diode, a drive circuit for controlling the light wavelength of the laser diode, a directional coupler for multiplexing / demultiplexing the output light of the laser diode, and an output light branched into two by the directional coupler. Then, a pair of photodetectors operating in a high-frequency band, a phase comparator that compares the phases of the respective high-frequency signals detected by the photodetectors, and a phase difference between the high-frequency signals input to the phase comparator. And a feedback circuit for controlling the driving circuit based on the phase difference detected by the phase comparator.

An optical transmitter according to the present invention is an optical transmitter using heterodyne detection of two lights used in optical subcarrier transmission, wherein the optical transmitter has a pair of optical wavelengths separated by a frequency interval of the optical subcarrier signal. , A driving circuit for controlling the light wavelength of the laser diode, a first directional coupler for multiplexing / demultiplexing the output light of the laser diode, and two branches by the first directional coupler. And a pair of photodetectors operating in a high-frequency band, a phase comparator for comparing the phases of respective high-frequency signals detected by the photodetectors, and input to the phase comparator. Delay means for providing a phase difference to the high-frequency signal, and of the output light branched into two by the first directional coupler, the output light on the side not provided with the delay means is input, and the output light of the optical transmitter is Second direction Vessels and and is characterized by comprising a feedback circuit for controlling the drive circuit by the detected phase difference by the phase comparator.

Further, by providing an optical intensity modulator for modulating the intensity of the optical signal from the directional coupler, the optical intensity can be modulated by the data signal.

Further, the laser diode can have a multi-electrode structure. By using a laser diode having a multi-electrode structure, the wavelength can be varied while maintaining a constant optical output. Furthermore, it is also possible to configure an optical transmitter module by hybridly integrating optical transmitters.

[0017]

Embodiments of the present invention will be described below in detail with reference to the drawings.

FIG. 1 is a diagram showing an embodiment (first embodiment) of an optical transmitter according to the present invention, wherein reference numerals 1a and 1b denote a pair of laser diodes (LD), 2 denotes a drive circuit, 3a, 3b is a directional coupler, 4a and 4b are a pair of photodetectors, 5 is a phase comparator, 6 is a feedback circuit, and 7 is a delay line.

The pair of laser diodes 1a and 1b have optical wavelengths separated by the frequency interval of the optical subcarrier signal, and the driving circuit 2 controls the optical wavelength. The directional couplers 3a and 3b combine and demultiplex the output lights of the laser diodes 1a and 1b, and a pair of photodetectors 4a and 4b that operate the output light branched into two in a high frequency band.
4b. Phase comparator 5
Is configured to compare the phases of the respective high-frequency signals detected by the photodetectors 4a and 4b, and control the drive circuit via the feedback circuit 6 based on the detected phase difference.
The delay line 7 adds a phase difference to the high-frequency signal input to the phase comparator 5.

Each of the LDs 1a and 1b oscillates in a single mode, and the difference between the two oscillation wavelengths corresponds to the high-frequency frequency interval. The drive circuit 2 controls the drive current and the temperature. The directional coupler 3a has two inputs and two outputs, and the directional coupler 3b has one input and two outputs. Both of the directional couplers 3a and 3b have a branching ratio of 1: 1. A delay line 7 is provided between the output side of the directional coupler 3b and the optical path of the photodetector 4b.
Are provided, and the photodetectors 4a and 4b and the phase comparator 5
Are connected by electric wires of the same length.
Double balance mixer (DBM) as the phase comparator 5
Is used.

With such a configuration, light generated from the LDs 1a and 1b is multiplexed / demultiplexed by the directional coupler 3a, one of which is output from the optical transmitter and the other is input to the directional coupler 3b. Only one output light of the output light branched into two by the directional coupler 3b passes through the delay line 7 and the photodetector 4
a and 4b. Photodetectors 4a, 4b
Is guided to the high frequency input terminal of the phase comparator 5 and the phases are compared. The phase comparator 5 outputs phase difference information between a high frequency signal approximately twice as high as the predetermined frequency signal and a low frequency signal. This output signal is supplied to the feedback circuit 6
And only the phase difference information of the low frequency signal is extracted and sent to the drive circuit 2 as feedback information. The drive circuit 2 corrects the oscillation wavelength by controlling the temperature, the drive current or both so that the frequency interval between the LDs 1a and 1b becomes equal to a predetermined high-frequency angular frequency.

In order to understand the frequency control operation, an optical electric field and a current on each line will be described. LD1a,
1b, the output optical electric fields E ₁ and E ₂ are

[0023]

(Equation 1)

[0024]

(Equation 2)

## EQU2 ## Here, A ₁ and A ₂ are the electric field amplitudes of the LDs _{1 a} and ₁ b, respectively, ω ₁ and ω ₂ are the angular frequencies, φ
₁ and φ ₂ are the phases of light. These two lights are multiplexed / demultiplexed by the directional couplers 3a and 3b and input to the photodetectors 4a and 4b. Optical field E ₁₁ is input to the optical detector 4a is

[0026]

(Equation 3)

## EQU1 ## The optical electric field E ₁₂ input to the photodetector 4 b is delayed by ΔT by the delay line 7,

[0028]

(Equation 4)

## EQU1 ## The output high-frequency current detected by the photodetectors 4a and 4b is proportional to the square of the input optical electric field,

[0030]

(Equation 5)

[0031]

(Equation 6)

## EQU1 ## Since the phase comparator 5 is a DBM, only the AC component of the third term in equations (5) and (6) is multiplied. That is, the alternating currents of the photodetectors 4a and 4b are set to I ₁₁ . Assuming I ₁₂

[0033]

(Equation 7)

[0034]

(Equation 8)

And the output current of the phase comparator 5 is the product of the two,

[0036]

(Equation 9)

## EQU3 ##

The feedback circuit 6 extracts the value of the cosine function cos (ω ₂ −ω ₁ ) ΔT included in the term of the DC component in equation (9) and uses the value as feedback information. Here, the effective length of the delay line 7 is set to a natural number multiple of 1/4 wavelength of the desired subcarrier high frequency. That is, assuming that the high-frequency angular frequency is ω ₀ , the delay time ΔT = (n / 4) (2π / ω ₀ )
(N is a natural number). Then cosω ₀ ΔT = cos
(Nπ) / 2, and when n is an odd number, cosω ₀ ΔT = 1, n
Is an integer, cosω ₀ ΔT = 0. Therefore, in order to synchronize the angular frequency interval ω ₂ −ω ₁ of the two lights with the desired angular frequency ω ₀ , the effective length of the delay line 7 is set to an odd multiple of １ ／ wavelength of the desired subcarrier high frequency. , Cos (ω ₂ −ω ₁ )
Feedback may be performed so as to maintain ΔT = 0. If the effective length of the delay line 7 is set to an even multiple of １ ／ wavelength of the desired subcarrier high frequency, cos (ω ₂ −
ω ₁ ) Feedback may be performed so as to maintain ΔT = 1.

The LDs 1a and 1b are distributed feedback semiconductor lasers (DFBs) having an oscillation wavelength of 1.55 micron band.
ted Feedback) laser is used. In order to use it as a 60 GHz high frequency optical transmitter, the frequency interval between the LDs 1a and 1b is set to 60 GHz. Since the high-frequency input for normally driving the phase comparator 5 was 0 dBm or more, the LD 1a,
The light output of 1b is +13 dBm. A quartz single mode fiber having a refractive index n of 1.5 is used for the optical path and the delay line 7. The length of the delay line 7 is set to 0.83 mm corresponding to 波長 wavelength of 60 GHz. High-speed photodiodes responding to 60 GHz are used for the photodetectors 4a and 4b. The optical inputs to the photodetectors 4a and 4b are both +10 dBm.

By the above operation, an optical subcarrier signal having good frequency stability is generated from the optical transmitter. The output of the optical transmitter is about +13 dBm.

FIG. 2 shows another embodiment (second embodiment) of the optical transmitter according to the present invention.
Is a pair of LDs, 12 is a drive circuit, 13a and 13b are directional couplers, 14a and 14b are a pair of photodetectors, 15 is a phase comparator, 16 is a feedback circuit, and 17 is a delay line.

The light generated from the LDs 11a and 11b is multiplexed / demultiplexed by the directional coupler 13a, one of which is input to the directional coupler 13b, and the other is directly guided to the photodetector 14b via the delay line 17. One of the two outputs of the directional coupler 13b is output from the optical transmitter, and the other is input to the photodetector 14a. Photodetectors 14a, 14
The high-frequency signals detected at b are guided to the high-frequency input terminal of the phase comparator 15 and are compared in phase. The output signal of the phase comparator 15 is input to a feedback circuit 16, and the feedback information extracted by the feedback circuit 16 is
Sent to The drive circuit 12 corrects the oscillation wavelength so that the frequency interval between the LDs 11a and 11b becomes equal to the desired high-frequency angular frequency ω ₀ . That is, the difference from the first embodiment shown in FIG. 1 is that the output signal of the directional coupler 13a is directly guided to the photodetector 14b via the delay line 17. The performance and driving conditions of the other elements are the same as in the first embodiment.

The light input to the photodetector 14b increases by 3 dB by not passing through the directional coupler 13b, and the high-frequency input to the phase comparator 15 becomes +6 dBm. Thereby, the diode of the double balance mixer is driven with more sufficient power, and a better output of the phase comparator can be obtained.

With such a loop configuration, an optical subcarrier signal having a stable frequency interval is transmitted from the output of the optical transmitter of the directional coupler 13b. The output of the optical transmitter is about +10 dBm.

FIG. 3 is a diagram showing still another embodiment (third embodiment) of the optical transmitter according to the present invention. In the figure, reference numeral 21
a and 21b are a pair of LDs, 22 is a drive circuit, and 23a and 2
3b is a directional coupler, 24a and 24b are a pair of photodetectors, 25 is a phase comparator, 26 is a feedback circuit, 27 is a delay line, and 28 is a light intensity modulator.

The light generated from the LDs 21a and 21b is split by the directional coupler 23a, and one output light is input to the light intensity modulator 28, which becomes the output of the optical transmitter after the intensity modulation. The other output light is input to the directional coupler 23b. Only one output light of the two branched output lights passes through the delay line 27, and the photodetectors 24a and 24b respectively.
Is input to The high-frequency signals detected by the photodetectors 24a and 24b are guided to the respective high-frequency input terminals of the phase comparator 25 and are compared in phase. The output signal of the phase comparator 25 is input to the feedback circuit 26, and the feedback information extracted by the feedback circuit 26 is sent to the drive circuit 22. The drive circuit 22 corrects the oscillation wavelength so that the frequency interval between the LDs 21a and 21b becomes equal to the desired high-frequency angular frequency ω ₀ . That is, the difference from the first embodiment shown in FIG. 1 is that the light intensity modulator 28 is provided on the output side of the directional coupler 23a. The performance and driving conditions of the other elements are the same as in the first embodiment.

The light intensity modulator 28 is a Mach-Zehnder type light modulator using a LiNbO ₃ substrate. Insertion loss is 5d
B, and the bias point used was a voltage at which the transmission characteristics became 50%. The speed of the input data signal is 2.5 Gbi
t / s.

With such a loop configuration, a stable and frequency-modulated optical subcarrier signal having a +5 dBm optical output from the output of the optical transmitter of the optical intensity modulator 28 was transmitted.

Further, in the configuration of the second embodiment of the present invention, it is also possible to connect an output signal of the directional coupler 13b to an optical intensity modulator and to perform optical intensity modulation by data as in the third embodiment. It is.

Further, since the amount of delay of the high frequency inputted to the phase comparator is adjusted by the sum of the optical path length difference and the wire length difference, instead of providing the delay line on the optical path, the phase comparison with the photodetector is made. A delay line may be provided on the electric wire between the devices, or both the optical path and the electric wire may have a difference. Further, a DFB laser having a multi-electrode structure can be used for the LD (for example, Y.
Yoshikuni, K. Oe, G. Motosugi, T. Matsuoka. "Broad wavele
ngth tuning under single-mode oscillation with am
ulti-electrode distributed feedback laser ", Electro
n. Lett., vol. 34, pp. 668-669, 1998). In this case, since the wavelength can be changed without changing the injection current, it is possible to keep the optical output constant. for that reason,
A change in beat output due to a difference in light output between the two lights can be suppressed.
Further, since the injection current to the LD is constant, the temperature does not change in the element, so that the temperature controllability of the drive circuits of the two LDs can be eased.

The optical transmitter can be made into one chip by hybrid-integrating each element on a quartz waveguide substrate or the like. At this time, the number of components is reduced by monolithically integrating a plurality of elements such as an LD, a photodetector, and a phase comparator, so that the mounting cost can be reduced. Further, the desired high-frequency angular frequency ω ₀ can be easily changed by changing the length of the delay line.

[0052]

As described above, according to the present invention, the output lights of a pair of laser diodes are multiplexed / demultiplexed, and the phase of an optical subcarrier signal output by two-optical heterodyne detection is compared with the reference high-frequency signal. In order to control the oscillation wavelength difference of the laser diode to a predetermined frequency interval of the optical subcarrier signal, the reference high-frequency signal is a delayed signal that is delayed after extracting a part of the optical subcarrier signal. By adding a phase difference to each optical subcarrier high-frequency signal input to the phase comparator, phase difference information can be extracted from the output signal of the phase comparator and used as feedback information for wavelength control. It becomes unnecessary, and it is possible to realize an optical transmission method using heterodyne detection of two lights having a stable frequency interval with a small configuration.

Also, by selecting the effective length of the delay line to be an arbitrary natural number times 1/4 wavelength of the optical subcarrier high frequency,
The frequency interval can be controlled more accurately.

Further, a pair of laser diodes having optical wavelengths separated by the frequency interval of the optical subcarrier signal, a driving circuit for controlling the optical wavelength of the laser diode, and a directional coupling for multiplexing / demultiplexing the output light of the laser diode. And a pair of photodetectors operating in a high-frequency band, respectively, and a phase comparator for comparing phases of respective high-frequency signals detected by the photodetectors. And a delay means for giving a phase difference to the high-frequency signal input to the phase comparator, and a feedback circuit for controlling the drive circuit based on the phase difference detected by the phase comparator, without using a high-frequency oscillator. By using heterodyne detection of two lights having a stable frequency interval, it is possible to realize an optical transmitter that is reduced in size.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating an embodiment (first embodiment) of an optical transmitter according to the present invention.

FIG. 2 shows another embodiment of the optical transmitter according to the present invention (second embodiment).
FIG.

FIG. 3 is a diagram showing still another embodiment (third embodiment) of the optical transmitter according to the present invention.

FIG. 4 is a diagram illustrating a configuration of a conventional optical transmitter.

[Explanation of symbols]

1a, 1b, 11a, 11b, 21a, 21b LD 2, 12, 22 Drive circuit 3a, 3b, 13a, 13b, 23a, 23b Directional coupler 4a, 4b, 14a, 14b, 24a, 24b Photodetector 5, 15, 25 Phase comparator 6, 16, 26 Feedback circuit 7, 17, 27 Delay line 28 Light intensity modulator

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) H01S 5/40 (72) Inventor Tetsuichiro Ohno 2-3-1 Otemachi, Chiyoda-ku, Tokyo Nippon Telegraph and Telephone Inside (72) Inventor Hiroshi Matsuoka 2-3-1 Otemachi, Chiyoda-ku, Tokyo F-term in Nippon Telegraph and Telephone Corporation (reference) 2K002 AA02 AB19 5F073 EA15 EA29 FA01 GA13 5K002 AA01 BA02 BA04 BA05 BA13 CA05 CA07 CA16 CA17 DA21

Claims (4)

[Claims] An output light of a pair of laser diodes is multiplexed / demultiplexed, an optical subcarrier signal output by two-optical heterodyne detection is compared in phase with a reference high-frequency signal, and an oscillation wavelength difference of the laser diode is determined by a predetermined value. An optical transmission method for controlling the frequency interval of an optical subcarrier signal, wherein the reference high-frequency signal is a delayed signal that is delayed after extracting a part of the optical subcarrier signal. 2. The optical transmission method according to claim 1, wherein the delay signal is a natural multiple of １ ／ wavelength of the frequency of the optical subcarrier signal. 3. An optical transmitter using heterodyne detection of two lights used in optical subcarrier transmission, comprising: a pair of laser diodes having optical wavelengths separated by a frequency interval of the optical subcarrier signal; A drive circuit for controlling the light wavelength of the laser diode, a directional coupler for multiplexing / demultiplexing the output light of the laser diode, and a pair of input lights which are respectively branched by the directional coupler and operate in a high frequency band. A photodetector, a phase comparator for comparing the phases of respective high-frequency signals detected by the photodetector, a delay means for giving a phase difference to the high-frequency signal input to the phase comparator, A feedback circuit for controlling the drive circuit based on a phase difference detected by the optical transmitter. 4. An optical transmitter using heterodyne detection of two lights used in optical subcarrier transmission, comprising: a pair of laser diodes having optical wavelengths separated by a frequency interval of the optical subcarrier signal; A drive circuit for controlling the light wavelength of the first, a first directional coupler for multiplexing / demultiplexing the output light of the laser diode, and the output light branched into two by the first directional coupler, respectively. A pair of photodetectors operating in a high-frequency band, a phase comparator for comparing the phases of the respective high-frequency signals detected by the photodetectors, and a delay for giving a phase difference to the high-frequency signal input to the phase comparator Means, and a second directional coupler which receives, as an input, output light on the side on which the delay means is not provided, and outputs the output light of the optical transmitter, out of the output light branched into two by the first directional coupler. Is detected by the phase comparator. And a feedback circuit for controlling a drive circuit based on the output phase difference.