JPH06303195A - Optical transmission device and optical receiver - Google Patents

Abstract

PURPOSE:To suppress the phase noise of a comparatively large laser in a transmission side. CONSTITUTION:The light from a laser 12 is distributed to two by an optical distributor 25, a phase modulation is performed for the one light by a base band signal by an optical modulator 13 and the light is made incident to an optical fiber 15. The other light from the optical distributor 25 is defined as reference light and it is made incident on the optical fiber 15. In a reception side, the modulation light from the optical fiber 15 and the reference light from an optical fiber 26 is synthesized by an optical synthesizer 19, the synthesized light is supplied to an optical detector 12, a homodyne detection is performed for the modulation light by the reference light, and a base band signal is reproduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to phase modulation of laser light with a baseband signal such as an analog signal or a digital signal, the modulated light is transmitted using an optical fiber, and the transmitted modulated light is used as a baseband signal. The present invention relates to an optical transmission system that reproduces the.

[0002]

2. Description of the Related Art As a method of converting a baseband signal into light and transmitting the light, there is a coherent optical transmission method. FIG. 4 shows a coherent optical transmission system in which the phase of light is modulated with a baseband signal. In the optical transmitter 11 on the transmission side, the phase of the light generated by the laser 12 is modulated by the baseband signal from the input terminal 14 using the optical modulator 13. The modulated light emitted from the optical modulator 13 is transmitted to the destination (reception side) through the optical fiber 15. In the optical receiver 16 on the receiving side, the light from the optical fiber 15 is split into two by the optical splitter 17,
From one of them, the optical carrier regenerator 18 reproduces the optical carrier,
With this reproduced optical carrier, the other light from the optical distributor 17 is homodyne detected by the photodetector 19 to reproduce the baseband signal and output it to the output terminal 22.

[0003]

Such optical coherent transmission can enable large-capacity transmission by wavelength division multiplexing in the optical region. However, in order to suppress the noise level of the signal reproduced on the receiving side to a low level, it is necessary to use a highly stable laser with extremely little phase noise and amplitude noise on the transmitting side. In addition, the optical carrier regenerator 18
As for the laser used for the same, it is necessary to prepare a highly stable laser as well as the transmitting side. Achieving such a highly stable laser is generally difficult and very expensive.

As described above, in the conventional coherent optical transmission system, when the baseband signal is transmitted while suppressing the noise level to a low level, it is necessary to use a highly stable and expensive laser. The problem to be solved by the present invention is to enable transmission of a baseband signal with low noise even when a laser having large amplitude noise and phase noise is used.

[0005]

In order to solve such a problem, according to the present invention, a part of laser light is branched on the transmitting side, and modulated light is transmitted by using this branched light as reference light. An optical fiber for reference light provided independently of the optical fiber is used for transmission, and a baseband signal is reproduced on the receiving side by using this reference light.

In the invention of claim 1, the modulated light from the optical fiber for modulated light and the reference light from the optical fiber for reference light are combined by the optical combiner on the receiving side, and the combined light is supplied to the photodetector. . According to the invention of claim 2, in the invention of claim 1, one of the modulated light and the reference light incident on the optical combiner is shifted in phase by a variable optical phase shifter, and a DC component is output from an output of the photodetector by a low-pass filter. Is detected, and the phase shift of the variable optical phase shifter is adjusted so that the detected output becomes zero.

According to the third aspect of the present invention, the modulated light from the modulation optical fiber and the reference light from the reference light optical fiber at the receiving side are combined with the light from the local oscillation laser by the first and second optical combiners, respectively. The combined light from the first and second optical combiners is combined and converted into a modulated intermediate frequency electric signal and a reference intermediate frequency electric signal by the first and second photodetectors, respectively, and these modulated intermediate frequency electric signal and reference Each amplitude of the intermediate frequency electric signal is limited to a constant value by the first and second amplitude limiters, and the phase of the output signal of the first amplitude limiter is detected by using the output signal of the second amplitude limiter as a reference phase. The phase is detected by the instrument.

[0008]

On the receiving side, the reference light and the modulated light are obtained from the same laser, the noise components contained in these are the same, and the detector can cancel the noise components of the reference light and the modulated light. The band signal can be reproduced with extremely low noise.

[0009]

Embodiment (1) An embodiment of the claimed invention is shown in FIG.
The parts corresponding to those in FIG. 4 are designated by the same reference numerals. In the present invention, the light from the laser 12 on the transmission side is split into two by the optical splitter 25. One of the distributed lights is incident on the optical modulator 13, the phase of the light is modulated by the baseband signal, and the emitted modulated light is transmitted by the modulated light optical fiber 15. The other split light is the reference light optical fiber 26 as the reference light.
And is transmitted to the receiving side. On the receiving side, without using the optical carrier regenerator, the reference light from the reference light optical fiber 26 enters the optical combiner 19 and is combined with the modulated light from the modulated light optical fiber 15. The combined light is supplied to the photodetector 21 and the baseband signal is reproduced.
Hereinafter, it will be described that laser noise is suppressed according to this configuration.

The light v _c generated by the laser 12 can be expressed as v _c (t) = {A + A _n (t)} sin {ω _c t + φ _n (t)} (1). Here, A is the amplitude of light, A _n (t) is amplitude noise, ω _c is the frequency of light, and φ _n (t) is the phase noise of light. Laser light in the baseband signal in the optical modulator 13 is phase modulated, the emitted modulated light v _m is _{v m (t) = {A} + A n (t)} sin {ω c t + φ m (t) + φ n (t) } It can be expressed as (2). Here, φ _m (t) represents a phase modulation component due to the baseband.

In the optical receiver 16, the reference light v _r (t) from the reference light optical fiber 26 has a relative phase difference φ between the modulated light optical fiber 15 and the reference light optical fiber 26 in the equation (1). _o considering _{the, v r (t) = {} a + a n (t)} sin {ω c t + φ n (t) + φ o} (3) and expressed. With homodyne detection by the optical detector 21 the modulated light by using the reference light (2), the output signal v _d after detection _{is, v d (t) = v} m (t) v r (t) = _{[{A + A n (t)} } 2/2 ] _{cos {φ m (t) -φ} o} - _{[{A + A n (t)} } 2/2 ] _{cos {2ω c t + φ m} (t) + 2φ n (t ) + Φ _o } (4) If the component of the second term on the right-hand side is removed by a low-pass filter and φ _n << 1, φ _o = π / 2, then v _d (t) ≈ [{A + A _n (t)} ² / 2] φ _m (t) (5) The modulated phase component is reproduced. By thus transmitting the reference light through the independent optical fiber 26,
As shown in the equation (5), the phase noise component in the laser 12 on the transmitting side can be canceled, and the baseband signal with less noise component can be reproduced. (2) In order to reproduce the phase component of the light modulated by the baseband signal, the photodetector 19 multiplies the modulated light and the reference light as shown in the expressions (4) and (5). In this case, the phase difference φ _o between the modulated light optical fiber 15 and the reference light optical fiber 26 was selected to be π / 2. By doing so, the linearity of the phase is good, and the reproduced baseband signal is not distorted. However, there is a possibility that φ _o cannot be maintained at π / 2 on the receiving side due to changes in the lengths of the fiber transmission lines 26 and 15 for the reference light and the modulated light.

This problem is solved by the invention of claim 2, and an embodiment thereof is shown in FIG. 2, and the portions corresponding to those in FIG. 1 are designated by the same reference numerals. In this embodiment, an optical phase shifter 27 for adjusting the phase rotation amount of the reference light from the reference light optical fiber 26 is inserted in the preceding stage of the optical combiner 19, and the phase shift amount of the optical phase shifter 27 is The DC component in the output of the photodetector 21 is adjusted to be zero. That is, the DC component in the output of the photodetector 21 is detected by the low-pass filter 28, and the amount of phase shift of the optical phase shifter 27 is controlled so that this DC component becomes zero, that is, the negative feedback is performed. In this way, φ _o can always be maintained at π / 2 even if the conditions such as the length of the optical fiber change. (3) The phase noise components of the laser 12 can be canceled by each of the embodiments shown in FIGS. However, the amplitude noise component A
_n (t) will remain. It is the invention of claim 3 that suppresses both the phase noise component and the amplitude noise component, and an embodiment thereof is shown in FIG. In this embodiment, a local oscillation laser 29 is provided in the optical receiver 16 on the receiving side, and the light from the local oscillation laser 29 and the modulated light from the modulated light optical fiber 15 and the reference light optical fiber 26 are emitted. The reference light is combined by the first light combiner 31 and the second light combiner 32, respectively, and the combined light of the first light combiner 31 and the second light combiner 32 is formed by the first light combiner, respectively.
The photodetector 33 and the second photodetector 34 convert the modulated intermediate frequency electric signal and the reference intermediate frequency electric signal. The amplitudes of the modulated intermediate frequency electric signal and the reference intermediate frequency electric signal are respectively the first amplitude limiter 35 and the second amplitude limiter 36.
The output signal of the first amplitude limiter 35 is phase-detected by the phase detector 37 using the output signal of the second amplitude limiter 36 as a reference phase to reproduce the baseband signal.

Light v ₁ (t) from the local oscillation laser 29
Let v _1n (t) be its phase noise and A _1n (t) be the amplitude noise, then v ₁ (t) = {A ₁ + A _1n (t)} cos {ω ₁ t + φ _1n (t)} (6) Can be represented. Here, ω ₁ is the frequency of the laser light for local oscillation. In the first photodetector 33 for the modulated light, the modulated light is converted into the modulated intermediate frequency electric signal v _IFm (t) having the intermediate frequency ω _{IF by} the local oscillation laser light represented by the equation (6). This signal v _IFm (t) is obtained by multiplying the equations (2) and (6) as _follows : v _IFm (t) = [{A + A _n (t)} {A ₁ + A _1n (t)} / 2] sin { ω _IF t + φ _m (t) + φ _n (t) −φ _{1 n} (t)} (7). Further, the reference light is also converted by the second photodetector 34 into the reference intermediate frequency electric signal v _IFr (t) having the intermediate frequency ω _IF as with the modulated light, and this signal v _IFr (t) is v _IFr. (t) = [{A + A _n (t)} {A ₁ + A _1n (t)} / 2] sin {ω _IF t + φ _n (t) + φ _o −φ _1n (t)} (8). (7) and (8) the signal V _IFm (t) and V _IFr represented respectively (t) is further in the first and second amplitude limiter 35, respectively, the amplitude becomes A _m and A _r The modulated intermediate frequency signal V _IFm (t) ′ and the reference intermediate frequency signal v _IFr (t) ′ that are limited in amplitude are V _IFm (t) ′ = A _m sin {ω _IF t + φ _m (t) + a _{φ n (t) -φ 1n (} t)} (9) V IFr (t) '= a r sin {ω IF t + φ m (t) + φ o -φ 1n (t)} (10). The first and second amplitude limiters 35 and 36 suppress the amplitude noises of the transmitter laser 12 and the receiver local oscillation laser 29. Further, the phase detector 37 _performs phase detection by multiplying each of the amplitude limited signals v _IFm (t) ′ and V _IFr (t) ′, and its detection output v _d (t) is v _d (t). = (A _m A _r / 2) cos {φ _m (t) −φ _o } (11). As a result, the phase noises of the transmitter laser 12 and the receiver local oscillation laser 29 included in the modulated intermediate frequency signal are canceled by the phase noise of the reference signal. Moreover, phi _o the wish to _{π / 2, φ m (t} ) «1 to the _{v d (t) ≒ (A} m A r / 2) φ m (t) (12) , and the modulated phase component, That is, the baseband signal is detected.

As described above, on the receiving side, the modulated light and the reference light are converted to the intermediate frequency by the same local oscillator, and the phase detection is further performed through the amplitude limiter to obtain a baseband signal with very little phase noise and amplitude noise. Can be transmitted.
Since the modulated light and the reference light are converted to the intermediate frequency,
The amplitude limiters 35 and 36 can be easily realized by an electric circuit.

In FIG. 2, the optical transporter 27 may be inserted not on the reference light side but on the modulated light side. The optical receiver 16 in FIG. 2 is the embodiment of claim 4, and the optical receiver 1 in FIG.
6 is an embodiment of claim 5.

[0016]

As described above, according to the present invention, an optical fiber for reference light is provided separately from the optical fiber for modulated light, the laser light used for modulation is distributed, and one of them is used as the reference light. The phase noise of the laser can be suppressed by transmitting with the reference light optical fiber and performing homodyne detection of the modulated light with the reference light on the receiving side. According to the invention of claim 3, not only the phase noise but also the amplitude noise is suppressed. The signal-to-noise ratio can be made extremely high. Further, as compared with the case where the optical carrier regenerator having a complicated structure has been conventionally required, the noise can be detected with a simple structure.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the invention of claim 1;

FIG. 2 is a block diagram showing an embodiment of each invention of claims 2 and 4;

FIG. 3 is a block diagram showing an embodiment of each invention of claims 3 and 5;

FIG. 4 is a block diagram showing a conventional optical transmission device.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Internal reference number FI Technical display location H04B 1/30 9298-5K 10/18 H04J 14/02 9372-5K H04B 9/00 E

Claims (5)

[Claims] 1. A transmitter side performs phase modulation of light generated by a laser with a baseband signal in an optical modulator, the modulated output light is transmitted by an optical fiber for modulated light, and the optical fiber for modulated light on the receiving side. Optical transmission for reconstructing the baseband signal by synthesizing the modulated light and the carrier light transmitted by the optical combiner, injecting the combined light into the photodetector, and homodyne detecting the modulated light with the carrier light. In the device, an optical distributor that distributes the light generated by the laser into a reference light and a modulating light and supplies the latter to the optical modulator, and the reference light is incident on the transmission side, and the reference light An optical fiber for reference light, which is transmitted to the receiving side and is supplied to the optical combiner as the carrier light. 2. A variable optical phase shifter capable of variably shifting one of the phases of the modulated light and the reference light incident on the optical combiner, and the output of the photodetector, The optical transmission device according to claim 1, further comprising a low-pass filter that detects the DC component and adjusts the phase shift amount of the variable optical phase shifter so that the detected output is zero. 3. A laser, an optical distributor for distributing the light generated by the laser into a modulating light and a reference light, an optical modulator for phase-modulating the modulating light with a baseband signal, and an optical modulation thereof. The modulated light output from the detector is input, and the modulated optical fiber that transmits it, the reference light that receives and transmits the reference light, the local oscillation laser, the local oscillation laser, and the local oscillation laser A first optical combiner for combining the light generated in 1. with the modulated light transmitted by the modulated light optical fiber; the light generated by the local oscillation laser and the reference light transmitted by the reference optical fiber. And a first photodetector for converting the combined light from the first optical combiner into a modulated intermediate frequency electric signal, and the combined light from the second optical combiner as a reference intermediate frequency electric signal A second photodetector for converting to A first amplitude limiter for limiting the amplitude of the modulated intermediate frequency electric signal to a constant value, a second amplitude limiter for limiting the amplitude of the reference intermediate frequency electric signal to a constant value, and an output of the first amplitude limiter An optical transmission device comprising: a phase detector that detects the phase of a signal using the output signal of the second amplitude limiter as a reference phase to reproduce the baseband signal. 4. An optical combiner for combining the modulated light from the modulated light optical fiber and the reference light from the reference light optical fiber, and one of the modulated light optical fiber and the reference light optical fiber. A variable optical phase shifter that is inserted in series between the optical combiner and shifts the phase of the optical signal, and the combined light is incident from the optical combiner, and the modulated light is homodyne-detected by the reference light to generate a baseband signal. A photodetector that regenerates the optical receiver, and a low-pass filter that detects the DC component from the output of the photodetector and adjusts the phase shift amount of the variable optical phase shifter so that the DC component becomes zero. Machine. 5. A local oscillation laser, a first optical combiner for combining the light generated by the local oscillation laser and the modulated light from the modulated light optical fiber, and the light generated by the local oscillation laser. A second light combiner for combining the reference light from the reference light optical fiber, a first photodetector for converting the combined light from the first light combiner into a modulated intermediate frequency electric signal, and a second light combiner Second photodetector for converting the combined light of 1. into a reference intermediate frequency electric signal, a first amplitude limiter for limiting the amplitude of the modulated intermediate frequency electric signal to a constant value, and a constant amplitude of the reference intermediate frequency electric signal A second amplitude limiter for limiting the value to a value, and a phase detector for detecting the phase of the output signal of the first amplitude limiter using the output signal of the second amplitude limiter as a reference phase to reproduce a baseband signal. An optical receiver comprising: