JPH0746191A - Optical phase synchronizing device - Google Patents

Abstract

PURPOSE:To perform phase synchronization even for phase fluctuation in a high frequency band without requiring a laser with complicated structure by finding light frequency difference between a master laser beam and a slave laser beam by detecting the interference signals of them by a photodetector, modulating the slave laser beam by the light frequency difference, and taking out a specific light frequency component. CONSTITUTION:When the slave laser beam outputted from a slave laser 2 is phase-synchronized with the master laser beam outputted from a master laser 1, the slave laser beam outputted from the slave laser 2 is separated by a partial transmission mirror 3b, and the beam on one side is multiplexed with the master laser beam by a partial transmission mirror 3a, then, it is made incident on the photodetector 4. The slave laser beam on the other side, to which delay being given by a movable mirror 8, is made incident on a phase modulator 7. The slave laser beam made incident on the phase modulator 7 is generated at the photodetector 4, and phase modulation is applied to it by a photoelectric current waveform amplified by an amplifier 5, and a beam with specific frequency can be taken out by a light frequency filter 9.

Description

Detailed Description of the Invention

[0001]

The present invention is used in optical communication and optical measurement. In particular, the present invention relates to a coherent optical communication technology and an optical phase synchronization technology necessary for realizing a sensor technology that utilizes optical interference. More specifically, a master laser that oscillates at a single optical frequency, and a laser beam that is completely synchronized with the oscillation frequency or phase of the master laser from a laser different from the master laser (this is called a slave laser). The present invention relates to a phase synchronizer.

[0002]

2. Description of the Related Art FIG. 8 is a block diagram showing a conventional optical phase synchronizer. This device comprises a master laser 1 that oscillates at a single optical frequency, a slave laser 2, and a partial transmission mirror 3 that combines the outputs of the two lasers 1 and 2.
A photodetector 4 for detecting the combined light, an amplifier 5 for returning the output of the photodetector 4 to the slave laser 2 and a low-pass filter 6 are provided. With this configuration, when the oscillation frequency of the slave laser 2 is synchronized with that of the master laser 1, the output of the low pass filter 6 becomes maximum. Therefore, the optical phase synchronization is realized by feeding this back to the slave laser 2 and adjusting the oscillation frequency thereof.

In this device, the slave laser 2 is
It is necessary that the oscillation frequency can be changed by a signal from the outside. This can be achieved by controlling the injection current of a semiconductor laser, for example.
Also, in other lasers, it can be achieved by mechanically controlling the length of the resonator by piezo control or the like.

[0004]

However, the conventional optical phase synchronization device has the following problems.

First, in the semiconductor laser, when the oscillation frequency is controlled by adjusting the injection current, usually, not only the oscillation frequency but also the oscillation intensity changes due to the change of the injection current. The amplitude modulation component contained in such a slave laser is not preferable for use in a coherent optical communication device or the like. As a countermeasure, the amplitude modulation component is suppressed by dividing the electrode of the semiconductor laser and providing a region for controlling the oscillation frequency independently of the portion for controlling the oscillation intensity. However, in order to completely control the intensity and frequency independently in this way, it is necessary to improve the response of the semiconductor laser to an ideal one, and it is difficult to manufacture it with high yield and at low cost. It was

When mechanically controlling the length of the resonator, the mechanical response band of the resonator length is usually several tens of k.
Since it is limited to about Hz, it is difficult to control phase fluctuations faster than this. Therefore, it can only be used for very limited purposes.

The present invention solves such a problem, does not require a laser having a complicated structure such as an electrode-divided semiconductor laser, and can realize phase synchronization even for phase fluctuations in a high band. An object is to provide a phase synchronization device.

[0008]

An optical phase synchronizer of the present invention comprises a laser light source and means for phase-locking the output light of this laser light source (hereinafter referred to as "slave laser light") with a master laser light. In the optical phase synchronization device, the means for phase synchronization includes means for detecting an optical frequency difference between the slave laser light and the master laser light, and means for modulating the slave laser light by using the detected optical frequency difference as a modulation signal. The laser light source has an oscillation average frequency that is set apart from the average value of the line width of the master laser light and the line width of the slave laser light with respect to the average frequency of the master laser light. And the slave laser light have a delay time difference smaller than at least one of the reciprocal of the master laser light line width and the reciprocal of the slave laser light line width. Characterized by comprising an optical delay means for delaying the slave laser light.

Means may be provided for extracting a predetermined optical frequency from the output light of the modulating means.

When the master laser light is a modulated optical signal train, it is preferable that the detecting means includes means for removing unnecessary spectral components. This means may be performed at the electrical signal stage or at the optical stage.

[0011]

[Operation] The interference signal between the master laser light and the slave laser light is detected by the photodetector, and the optical frequency difference is obtained,
The slave laser light is modulated by the optical frequency difference. At this time, the time required to obtain the optical frequency difference after branching one slave laser beam to obtain the modulation signal and the delay time of the slave laser beam modulated by the modulation signal must satisfy a predetermined relationship. For example, a component synchronized with the master laser light is generated in the modulated output light. Furthermore, if the respective oscillation average frequencies of the master laser light and the slave laser light are far from the average value of the respective line widths, the component synchronized with the master laser light is separated from other components on the spectrum, and the optical frequency Can be separated by a filter.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a block diagram showing an optical phase synchronizer according to a first embodiment of the present invention. This apparatus includes a slave laser 2 as a laser light source, and means for phase-locking the slave laser light output from the slave laser 2 with the master laser light output from the master laser 1. Here, the feature of this embodiment is that the means for phase synchronization is provided with partial transmission mirrors 3a, 3b as means for detecting the optical frequency difference between the slave laser light and the master laser light.
The slave laser 2 includes a photodetector 4 and an amplifier 5, and a phase modulator 7 as a means for modulating the slave laser light by using the detected optical frequency difference as a modulation signal. The slave laser 2 has an oscillation average frequency of the master laser light. It is set apart from the average value of the line width of the master laser light and the line width of the slave laser light with respect to the average frequency, and the delay time difference between the modulation signal in the phase modulator 7 and the slave laser light is the line of the master laser light. The movable mirror 8 is provided as an optical delay unit that delays the slave laser light so as to be smaller than at least one of the reciprocal of the width and the reciprocal of the line width of the slave laser light. Moreover, an optical frequency filter 9 is provided as a means for extracting a predetermined optical frequency from the output light of the phase modulator 7.

The slave laser light output from the slave laser 2 is separated by the partial transmission mirror 3b, one of which is combined with the master laser light by the partial transmission mirror 3a and enters the photodetector 4. The other slave laser beam separated by the partial transmission mirror 3b enters the phase modulator 7 after being delayed by the movable mirror 8. The photocurrent generated in the photodetector 4 is amplified by the amplifier 5 and becomes a drive signal for the phase modulator 7. The slave laser light that has entered the phase modulator 7 is phase-modulated by the photocurrent waveform generated in the photodetector 4, and the light of a specific frequency is extracted by the optical frequency filter 9.

To explain this operation in more detail, the complex electric field amplitudes E _M (t) and E _S (t) of the master laser light and the slave laser light are expressed by the following equations.

[0015]

[Equation 1] Here, a _M and a _S represent amplitudes, and ν _M and ν _S are average optical frequencies of the respective laser lights. Here, the respective temperatures of the master laser 1 and the slave laser 2 are set so that the average optical frequency of the respective laser beams does not change.
It is necessary to control the injection current and other constants. θ _M
Although (t) and θ _S (t) are phases and change randomly with respect to time t, they are almost constant within the coherence time of the laser light and are represented as follows.

[0016]

[Equation 2] However, τ <T _M , τ <T _S , T _M and T _S are coherence times of the master laser light and the slave laser light, respectively, and laser line widths Δν _M and Δν _S equivalent to the band of phase fluctuation. And have the following relationship.

[0017]

[Equation 3] The photocurrent I (t) generated in the photodetector 4 is given by the following equation.

[0018]

[Equation 4] The first and second terms of this equation are DC components. Since it is sufficient to consider only the AC component as the drive signal of the phase modulator 7, the drive signal V (t) is as follows.

[0019]

[Equation 5] Here, after the slave laser light is separated by the partial transmission mirror 3b, the time required for the photocurrent signal of Expression 5 to reach the phase modulator 7 and the slave laser light directly arrives at the phase modulator 7. The difference from the time required for is represented by τ _d . At this time, the slave laser light phase-modulated by the signal of Expression 5 is expressed by the following expression.

[0020]

[Equation 6] This equation is expanded to the Fourier series as follows.

[0021]

[Equation 7] This equation shows that the output from the phase modulator 7 is the frequency ν _S + k
(Ν _M −ν _S ), where −∞ <k <∞, a large number of continuous lights are synthesized. The optical frequencies of these continuous lights are arranged at regular intervals ν _M −ν _S. Here, in the case of k = 1 among these, that is, attention is focused on light having an optical frequency of ν _M. The term of k = 1 is expressed by the following equation.

[0022]

[Equation 8] In this equation, if τ _d <T _S , from Equation 2, this light has exactly the same phase fluctuation as the master laser light. Therefore, if the condition of τ _d <T _S is realized, output light phase-locked with the master laser light can be obtained by extracting only the light of frequency ν _M from the output of the phase modulator 7 by the optical frequency filter 9. it can. τ _d <T
_In order to realize the state of _S, the optical path length of the slave laser light is adjusted by the movable mirror 8 in this embodiment. Assuming that the line width of the slave laser light is 100 MHz, T _S = 10 ns according to Formula 3, so that the optical path length is approximately cT _S =
It can be easily achieved by adjusting with an accuracy of 3 m (c is the speed of light).

In order to operate this embodiment, the following conditions need to be satisfied.

First, the photodetector 4, the amplifier 5 and the phase modulator 7 have an average optical frequency difference ν _M -ν of two laser beams.
It has a band in the vicinity of _S and must be larger than the phase fluctuation bands Δν _M and Δν _S.
However, Δν _M and Δν _S are at most 1 for semiconductor lasers.
It is about 00 MHz, and it is easy to realize such a band.

Second, it is necessary to set the average optical frequency difference ν _M −ν _S of the two laser beams to a certain degree.
The first reason for this is that when the average optical frequency difference ν _M −ν _S becomes too small, when a specific frequency is extracted from a large number of optical frequencies arranged at intervals of ν _M −ν _S as shown in Equation 7, the frequency becomes extremely high. That is, a high-resolution optical frequency filter is required. Therefore, the average optical frequency difference ν _M −
It is necessary to set ν _S larger than the resolution of the optical frequency filter used. The second reason is that if the instantaneous frequency of light corresponding to each value of k in Equation 7 overlaps with the average frequency of light corresponding to the next k, no matter what resolution the optical frequency filter has, the Will not be separated. The condition required for this is expressed by the following equation.

[0026]

[Equation 9] That is, ν _M −ν _S needs to be set larger than the average value of the line width of each laser beam. However, since the line width of the laser light is generally defined by the 3 dB lowering band, the extinction ratio of adjacent light is 3 dB under the critical condition of Expression 9, which is not practically sufficient. Practically, it is desirable to make it 10 times or more larger than the average of the line widths of the two laser beams. Therefore, in consideration of the first constraint, ν _M −ν _S is 1 to 1
It is considered desirable to set the frequency to about 0 GHz.

In this way, for a master laser that oscillates at a single optical frequency, a laser beam that is completely synchronized with the oscillation frequency or phase of the master laser can be obtained by a laser different from the master laser. You can

Although the phase modulator is used as the modulating means in this embodiment, the present invention can be implemented by using the amplitude modulator. However, k = 1 for the phase modulator
It is possible to increase the generation efficiency of the frequency with respect to.

FIG. 2 is a block diagram showing an optical phase synchronizer according to the second embodiment of the present invention. This embodiment is an optical signal train in which the master laser light is modulated, and a band pass filter 10 is provided between the amplifier 5 and the phase modulator 7 as means for removing unnecessary spectral components. Different from the example. In this case, the slave laser light is phase-locked with the optical carrier frequency of the optical signal train input as the master laser light.

3 and 4 are diagrams for explaining the operation of this embodiment. Regarding the operation of this embodiment, the difference from the first embodiment will be described.

Since the input in this embodiment is a modulated optical signal train, its spectrum is not monochromatic, but spreads around the optical frequency ν _{M of the} carrier as shown in FIG. The spectrum of the photocurrent when it enters the photodetector 4 together with the slave laser light spreads around ν _M −ν _S as shown in FIG. Therefore, the band pass filter 10 extracts only frequencies in the vicinity of ν _M −ν _S , and drives the phase modulator 7 accordingly. As a result, as in the first embodiment, it is possible to obtain light that is phase-locked with the optical frequency ν _{M of the} carrier.

FIG. 5 is a block diagram showing an optical phase synchronizer according to the third embodiment of the present invention. This embodiment is different from the second embodiment in that an optical frequency filter 11 is provided at the input stage as means for directly removing unnecessary spectral components from the master laser light.

6 and 8 are diagrams for explaining the operation of this embodiment. This embodiment is used when the spread of the spectrum of the optical signal train is larger than twice ν _M −ν _S as shown in FIG. The operation of this embodiment is almost the same as that of the second embodiment, but differs in the following points.

In order to drive the phase modulator 7, it is necessary to extract an interference signal between the slave laser light (optical frequency ν _S ) and the carrier of the optical signal train (optical frequency ν _M ), but the spectrum of the optical signal train is required. If the spread is greater than 2 times the [nu _M -v _S of the spectral component at the position of [nu _M and symmetrically with respect to the [nu _S is mixed in a position of the spectrum [nu _M -v _S photocurrent Will end up. To prevent this, the optical frequency filter 1
By 1, the spectral component at a position symmetrical to v _M with respect to v _S may be removed. As a result, the optical frequency ν of the carrier wave is exactly the same as in the second embodiment.
Light that is phase-locked with _M can be obtained.

[0035]

As described above, the optical phase-locking device of the present invention does not require a laser having a complicated structure such as an electrode-divided semiconductor laser, and realizes phase-locking with respect to a phase fluctuation in a high band. be able to. INDUSTRIAL APPLICABILITY The present invention is particularly effective in the fields of optical communication and optical measurement, particularly in coherent optical communication technology and sensor technology utilizing optical interference.

[Brief description of drawings]

FIG. 1 is a block configuration diagram showing an optical phase synchronization device of a first embodiment of the present invention.

FIG. 2 is a block diagram showing an optical phase synchronizer according to a second embodiment of the present invention.

FIG. 3 is a diagram for explaining the operation of the second embodiment and is a diagram showing a spectrum of an optical signal.

FIG. 4 is a diagram for explaining the operation of the second embodiment, showing the spectrum of the photocurrent signal output from the photodetector.

FIG. 5 is a block configuration diagram showing an optical phase synchronization device of a third embodiment of the present invention.

FIG. 6 is a diagram for explaining the operation of the third embodiment and is a diagram showing a spectrum of an optical signal.

FIG. 7 is a diagram for explaining the operation of the third embodiment, showing the spectrum of the photocurrent signal output from the photodetector.

FIG. 8 is a block diagram showing a conventional optical phase synchronizer.

[Explanation of symbols]

 1 Master Laser 2 Slave Laser 3, 3a, 3b Partial Transmission Mirror 4 Photo Detector 5 Amplifier 6 Low Pass Filter 7 Phase Modulator 8 Movable Mirror 9, 11 Optical Frequency Filter 10 Band Pass Filter

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Office reference number FI technical display location G02F 1/01 B

Claims (3)

[Claims] 1. An optical phase synchronizer comprising a laser light source and means for phase-locking the output light of the laser light source with a master laser light, wherein the means for phase-locking is the output light of the laser light source and the master. A means for detecting an optical frequency difference with the laser light, and a means for modulating the output light of the laser light source using the detected optical frequency difference as a modulation signal, wherein the laser light source has an oscillation average frequency of the master With respect to the average frequency of the laser light, set apart from the average value of the line width of the master laser light and the output line width of the laser light source, the modulation signal in the modulating means and the output light of the laser light source. Of the laser light source such that the delay time difference is smaller than at least one of the reciprocal of the line width of the master laser light and the reciprocal of the output line width of the laser light source. An optical phase synchronization device characterized by comprising an optical delay means for delaying the output light of the. 2. A means for extracting a predetermined optical frequency from the output light of the modulating means.
The optical phase synchronizer described. 3. The optical phase synchronizer according to claim 1, wherein the master laser light is a modulated optical signal train, and the detecting means includes means for removing unnecessary spectral components.