JPH02201323A - Optical heterodyne detector - Google Patents

Abstract

PURPOSE:To allow stable operation over a long period of time with simple constitution by generating the stimulated Brillouin scattered light in which polarized light is scrambled by using an optical fiber resonator and using this light as the locally oscillated light for optical heterodyne detection. CONSTITUTION:A locally oscillated light source has the optical fiber resonator 7 and a stimulating light source 5 which generates the stimulated Brillouin scattered light by stimulating this optical fiber resonator 7. The stimulated Brillouin scattered light generated by the optical fiber resonator 7 is used as the locally oscillated light. The polarized light of the locally oscillated light is scrambled in this way and, therefore, even if the polarization state of the signal light is changed in whatever way by optical fiber transmission, the magnitude of the differential frequency component between the stimulated Brillouin scattered light and the signal light is kept always constant. The need for adopting polarized light control and polarization diversity as in the conventional method is eliminated in this way and, therefore, the constitution is simple and the operation stable over a long period of time is assured.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an optical heterodyne detection device used in large-capacity, long-distance optical communications.

(Prior art) Optical communication using the optical heterodyne detection method has a reception sensitivity of 10 to 20% compared to that using the conventional optical direct detection method.
Because it has the characteristic of being about dB high, it is being studied as an effective detection method for long-distance optical communication trunk systems. In addition, when considering wavelength-multiplexed optical communication, the conventional optical direct detection method requires an optical demultiplexer to separate the signals of each wavelength. was limited by its wavelength resolution. On the other hand, in the optical heterodyne detection method, each signal can be separated in the electrical domain, so it is expected that extremely high-density wavelength multiplexed optical communication will become possible (for example, IE , IEEE Journal of Technology
Lightwave Technology) - Part L
Volume T-5 (1987), pages 414-428).

In this optical heterodyne detection method, a signal light transmitted through an optical fiber and a local oscillation light whose frequency is slightly different from this signal light are multiplexed and then input into an optical receiver. signal). Then, by demodulating this beat signal, the transmitted information signal is extracted.In order to obtain high reception sensitivity with this method, it is necessary to maximize the beat signal. It is necessary to match the polarization state with the local oscillation light. However, in general, the polarization state of signal light propagating through an optical fiber changes over time due to disturbances such as disturbances to the optical fiber and temperature fluctuations.

(Problem to be Solved by the Invention) As a method for solving this problem, there is a method that first monitors the polarization state of the signal light and uses this monitor signal to feedback control the polarization state of the signal light or local oscillation light. Are known. However, this feedback control method has a fatal drawback in that it cannot follow changes in the polarization state faster than the time constant of the feedback control. Furthermore, the polarization control device required for polarization control has the problem that, in addition to not being able to operate safely over a long period of time, the insertion loss is relatively large.

Another method for avoiding the effects of polarization state fluctuations is to separate the signal light transmitted through an optical fiber into two orthogonal polarization components, and combine each polarization component's signal light with local oscillation light. However, this polarization diversity method requires two receivers and two receiving circuits, complicates the demodulation circuit, and makes the device expensive. There was a drawback.

The purpose of the present invention is to eliminate the drawbacks of the prior art as mentioned above,
An object of the present invention is to provide an optical heterodyne detection device that has a simple configuration and operates stably over a long period of time.

(Means for Solving Problems) The optical heterogain detection device of the present invention includes a local oscillation light source that emits locally oscillated light, and an optical multiplexer that combines the signal light transmitted through an optical fiber and the locally oscillated light. a photodetector for detecting the difference frequency component between the two lights obtained by combining the signal light and the local oscillation light by the multiplexer, and demodulating the information signal from the difference frequency component. In the optical heterodyne detection device including a demodulation circuit, the local oscillation light source includes an optical fiber resonator and an excitation light source that excites the optical fiber resonator to generate stimulated Brillouin scattered light, and the optical fiber resonance A structural feature is that stimulated Brillouin scattered light generated in the device is used as the locally oscillated light.

(Function) First, stimulated Brillouin scattering in the optical fiber resonator used in the present invention will be explained.

Generally, when a single frequency laser beam (pumping light) is input into an optical fiber, the pumping light reaches 10 GH during propagation through the optical fiber.
It is observed that the light is converted into light with a frequency as low as z and is backscattered. This is a coherent light scattering phenomenon caused by acoustic phonons in an optical fiber, and is called stimulated Brillouin scattering. This stimulated Brillouin scattering can be induced with a low pumping input when using an optical fiber resonator due to its resonance effect.For example, consider an all-fiber ring resonator using an optical fiber coupler as shown in Figure 3. The excitation input (threshold) Pl at which stimulated Brillouin scattering begins to occur is approximately given by the following equation (Optics Letter
s), Vol. 7 (1982), pp. 509-511)
.

1+Ke-” p,=ps
...■1- However, Ae α el where α is the transmission loss of the optical fiber, l is the loop length of the ring resonator, K is the coupling coefficient from the port of the optical fiber coupler to 0, and Ae is The effective core cross-sectional area of the optical fiber, g is the stimulated Brillouin gain coefficient of the optical fiber (=4.587X1
0-''m/W).

Now, the entire fiber ring resonator mentioned above has an effective core diameter of 10
μm, fiber length 100m, transmission loss 0.2dB/km
Considering that it is composed of a single mode optical fiber of
, (2), it is predicted to be approximately 1.7 mW. Since this excitation input power level is sufficiently low, stimulated Brillouin scattered light can be easily generated by using a single frequency semiconductor laser as the excitation light source.

Here, when light is propagated using a single mode optical fiber, the polarization state of the light changes as it propagates through the optical fiber.

This change in the candlelight state has periodicity in the longitudinal direction of the optical fiber, and this period is called the beat length. The polarization state of light at this beat length takes the orthogonal B5 light, clockwise and counterclockwise elliptical dimming states, and can be considered to include all the candlelight states. Here, in order to realize the above polarization state in a polarization-maintaining optical fiber, it is sufficient to input light so that the principal axis direction of the optical fiber does not coincide with the polarization direction of the light.

On the other hand, stimulated Brillouin scattered light occurs in a direction opposite to the propagation direction of the excitation light and throughout the optical fiber. As a result, the polarization state of the stimulated Brillouin-scattered light becomes a replica of the polarization state that the excitation light can take after being corrected in the longitudinal direction of the optical fiber. Therefore, if the loop length of the optical fiber ring resonator mentioned above is made longer than the beat length of the optical fiber, the polarization states of the generated stimulated Brillouin scattered light will be orthogonal to each other! It contains both right-handed and left-handed elliptical polarization states and is equivalent to being completely scrambled.

In the present invention, stimulated Brillouin scattered light with such polarized light scrambled is used as locally oscillated light. Therefore, since the polarization of one local oscillation light is scrambled, no matter how much the polarization state of the signal light changes due to optical fiber transmission, the difference frequency component between the stimulated Brillouin scattering light and the signal light (
The magnitude of the beat signal) is always kept constant. As a result, in the present invention, there is no need to employ polarization control or polarization diversity as in the prior art, so it is possible to realize an optical heterodyne detection device that has a simple configuration and operates stably over a long period of time.

(Example) Next, an optical heterodyne detection device of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a block diagram of an embodiment according to the present invention, and FIG. 2 is a diagram showing the frequency relationship among signal light, excitation light, and stimulated Brillouin scattered light in this embodiment.

In FIG. 1, the semi-IH4C lasers 11 and 51 are
The oscillation wavelength is 1.55. czm InGaAsP/InP
Distributed feedback semiconductor laser, external modulator 21 is LiNbO
3 optical intensity modulator, optical fiber 3 has an effective core diameter of 8 μm,
A single mode optical fiber with a fiber length of 150 km and a transmission loss of 0.2 dB/km at a wavelength of 1.55 μm, optical multiplexer 4.6 is a single mode optical fiber coupler with a branching ratio of 1:1, and a light receiver. 8 uses an InGaAs avalanche photodiode (APD). The optical fiber ring resonator 7 has an M configuration similar to that shown in Fig. 3, and has an effective core diameter of 5 μm, a beat length of 10 mm, a fiber length of 50 m, and a polarization-maintaining single mode with a transmission loss of 0.25 dB/km. It consists of an optical fiber and a single mode optical fiber coupler with a coupling coefficient of =0.6. Here, in order to ensure that the polarization direction of the incident excitation light and the principal axis direction do not match in any case, the above-mentioned polarization preserving fiber has two polarization preserving fibers each having a length of 25 m. They are fusion spliced so that their directions differ by 45° from each other.

In this embodiment, the signal light emitted from the distributed feedback semiconductor laser 11 is transmitted to the electrical signal input terminal 22 of the L i N b Oi optical intensity modulator 21 at 100 Mb/
100 Mb by s binary code electric signal pulse
/s for amplitude shift keying (AS
K) Modulated. And this 100Mb/5AsK
The modulated signal light is transmitted by a single mode optical fiber.

On the other hand, the excitation light emitted from the distributed feedback semiconductor laser 51 is input into the optical fiber ring resonator 7 by the single mode optical fiber coupler 6. Here, the stimulated Brillouin scattering threshold of this optical fiber ring resonator was approximately 4 mW as calculated from the formula 00. In this example, the pumping input optical power to the optical fiber ring resonator was 7 mW. At this time, stimulated Brillouin scattered light of 1.6 mW was generated. Then, by the single mode optical fiber coupler 4, half of the 0.8 mW stimulated Brillouin scattered light is coupled to the InGaAs-AP along with the ASK modulated signal light that has propagated through the single mode optical fiber.
The light is received by D8.

In this example, by controlling the temperature of the semiconductor laser 51, the frequency of the excitation light was set to be 11.5 GHz higher than the frequency of the ASK modulated signal light. Here, in the wavelength band of 1.5 μm, the stimulated Brillouin scattered light is generated on the lower frequency side by 11 GHz than the excitation light. Therefore, the frequency relationship among the pump light, stimulated Brillouin scattered light, and ASK modulated signal light in this example is as shown in FIG.
8, a beat signal with a center frequency of 500 MHz is obtained. The demodulation circuit 91 amplifies this beat signal, demodulates it by envelope detection, and outputs an information signal from an electrical signal output terminal 92. Note that the receiving sensitivity in this example was 54 dBm, which was about 6 dB higher than direct optical detection using a similar photoreceiver.

Next, in order to confirm that this embodiment operates stably even if the polarization state of the signal light varies, an experiment was conducted in which vibration was applied to the single mode optical fiber 3.

As a result, it was confirmed that in this example, the signal could always be stably demodulated no matter how much vibration was applied.

Although the optical heterodyne detection device according to the present invention has been described above using one embodiment, the present invention is not limited to this embodiment, and several modifications can be made.
For example, as the semiconductor lasers 11 and 51, InGaA
Although sP, /InP distributed feedback semiconductor lasers are used, semiconductor lasers made of other materials, semiconductor lasers with external mirrors, solid-state lasers, gas lasers, and other types of lasers may also be used, and the wavelength range is not limited. Signal light modulation method%
Formula% (FSK) or Phase Shift Keying (P
It is clear that the modulation method (SK) may be used, and the bit rate may be arbitrary.

Furthermore, as the light receiver 8, it is possible to use an APD made of other materials, a photodiode, a photomultiplier tube, etc.
The optical fiber 3 and the optical fiber resonator 7 may be dispersion shifted fibers or optical fibers with other compositions such as Q e O2 or P2O2, and are not limited to the embodiments.

(Effects of the Invention) As explained above, in the optical heterogain detection device according to the present invention, stimulated Brillouin scattered light with scrambled polarization is generated using an optical fiber resonator, and this is used for optical heterodyne detection. It is used as local oscillation light. As a result, in the present invention, there is no need to employ polarization control or a polarization diversity method, so there is an advantage that an optical hederodyne detection device that has a simple configuration and operates stably over a long period of time can be realized.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of an embodiment according to the present invention, Fig. 2 is a diagram showing the frequency relationship of signal light, excitation light, and stimulated Brillouin scattered light in this embodiment, and Fig. 3 is a diagram showing the relationship between the frequencies of the signal light, excitation light, and stimulated Brillouin scattering light in this embodiment. FIG. 3 is a diagram showing the configuration. 1...Signal light source, 11.51...Semiconductor laser, 1
2.52... Lens, 13.53... Optical isolator, 21... External modulator, 22... Electric signal input terminal, 3... Optical fiber, 4.6... Optical multiplexer , 5・
・・Excitation light source, 7・・Optical fiber ring resonator, 8・
... Photodetector, 91 ... Demodulation circuit, 92 ... Electric signal output terminal, 100 ... Optical fiber coupler, 200
...optical fiber

Claims (1)

[Claims] A local oscillation light source that emits a local oscillation light, an optical multiplexer that multiplexes the signal light transmitted through an optical fiber and the local oscillation light, and the multiplexer combines the signal light and the local oscillation light. In the optical heterodyne detection device, the optical heterodyne detection device includes a photodetector that detects a difference frequency component between these two lights obtained by oscillation, and a demodulation circuit that demodulates an information signal from this difference frequency component. It comprises a resonator and an excitation light source that excites the optical fiber resonator to generate stimulated Brillouin scattered light, and the stimulated Brillouin scattered light generated in the optical fiber resonator is used as the local oscillation light. Optical heterodyne detection device.