CN110752882A - Low-bit-error-rate phase coding system and receiving end thereof - Google Patents

Abstract

The invention provides a phase coding system and a receiving end thereof, which are particularly suitable for distributing quantum keys. The receiving end can include unequal arm interferometer and detection module, and the interferometer comprises beam splitting unit, long arm, short arm, first and second reflection unit, wherein through setting the reflection unit to the polarization direction that can make the light pulse of reflection deflect 90 degrees, can solve polarization disturbance and transmission fiber polarization disturbance that the difference of the long short arm that is used for unequal arm interferometer that exists among the scheme that is used for phase coding among the prior art leads to effectively, the problem that the receiving count rate that brings descends.

Description

Low-bit-error-rate phase coding system and receiving end thereof

Technical Field

The invention relates to the field of communication, in particular to a phase coding system capable of providing high contrast and low bit error rate and a receiving end thereof, which are particularly suitable for quantum key distribution.

Background

Quantum secure communication is an emerging field of communication technology today, and is more the main development direction and trend of the communication technology today. Compared with the traditional communication, the method has incomparable great advantages: in principle, absolute security of the communication can be guaranteed. Therefore, researchers in various countries concentrate on the strength to develop quantum secret communication, so that the development of quantum secret communication is changed day by day.

In the field of quantum secure communications, the most widely used and mature direction is the quantum key distribution direction. The quantum key distribution is ensured by the basic principles of quantum irreproducibility and the like of quantum mechanics, and information is transmitted among national defense units, government offices, scientific research units, financial institutions and the like in a one-time pad encryption mode.

With the gradual expansion of applications, polarization encoding and phase encoding are more common in encoding schemes of quantum key distribution.

In 1984, Bennett-Brassard proposed a polarization encoding scheme. The scheme encodes information on different polarization states, uses single photons to carry and transmit, and is still the most important encoding scheme for quantum key distribution so far due to low insertion loss, low cost and simple structure. However, since this scheme is encoded using different polarization states of photons, the stability of the fiber transporting a single photon is of critical importance. However, the polarization disturbance of the fiber is greatly affected by the environment, thereby directly affecting the bit error rate. In the process of practical application, the environment of the optical fiber is various and comprises an aerial optical cable, so that a polarization feedback mechanism needs to be started frequently or a fast polarization module needs to be used for correcting polarization disturbance so as to control the error rate, and therefore, the code forming at a normal level is ensured. However, both the polarization feedback and fast polarization modules result in a waste of time dimension, i.e., the time per unit time for coding is reduced, so that the phase change limits the improvement of the coding rate and increases the instability factor.

With respect to polarization-encoded schemes, phase-encoded schemes use an unequal arm interferometer to split a light pulse generated by a light source into two light pulses, one before the other, and modulate the relative phase difference between the two pulses to encode and carry information. The advantage of the phase-coded scheme is that two pulses before and after each time are used to carry the information carried by one pulse in the original polarization code. Even if the phase of the optical pulse pair is influenced by the environment in the optical fiber transmission process, the relative phase difference of the optical pulse pair is less influenced by the environment because the phase change caused by the environment is the same for the front pulse and the rear pulse. Even if the received counting rate is reduced due to the change of polarization, the error rate is not increased. This makes the phase encoding scheme more suitable for situations where the polarization changes are more severe.

However, in practical applications, the optical pulse in the phase encoding scheme needs to pass through the unequal-arm interferometer, and is divided into two beams by the beam splitter, and the two beams respectively pass through the long arm and the short arm of the unequal-arm interferometer. The difference between the long and short arms results in the light pulses therein having different environments with respect to each other, which causes the two light beams to differ in phase. The difference is reflected in the fast and slow axes of the optical fiber, and the phase difference between the light of the long-arm fast axis and the light of the short-arm fast axis is different from the phase difference between the light of the long-arm slow axis and the light of the short-arm slow axis, so that when the long-arm light and the short-arm light are required to be subjected to interference decoding (as in a traditional phase decoding scheme), the interference effect of the long-arm fast axis light and the short-arm fast axis light is different from the interference effect of the long-arm slow axis light and the short-arm slow axis light. This different interference effect results in a decrease in interference contrast for the sender and receiver and thus an increase in system error rate. To ensure high contrast of the interference, the polarization maintaining coupler of the unequal-arm interferometer in the receiver can adopt a fast axis cut-off mode to discard photons entering the fast axis. In order to reduce the number of dropped photons, the polarization change in the optical fiber transmission process must be feedback corrected, and additional devices or lines are required, which leads to increased cost and complicated operation.

Several solutions have also emerged in the prior art to address this problem. Fig. 1 shows a receiving end structure for a phase encoding scheme in the prior art. As shown in the figure, in the receiving end, a polarization controller is added, while the unequal arm interferometer is set as a polarization maintaining unequal arm interferometer, and in the interferometer, the fast axis or the slow axis (for the optical pulse) of the optical fiber is cut off. The phase change of the fast and slow axes causes different interference effects of the long/short arm fast and slow axes, so that the fast axes of the long arm and the short arm are cut off or the slow axes of the long arm and the short arm are cut off. Accordingly, under the structure, the polarization controller can be used for aligning the polarization direction of the incident light to the non-cut-off optical axis, so that the long-arm and short-arm optical interference effect is kept stable, and optionally, a phase modulation device such as a phase shifter can be combined to compensate the phase drift of the long arm and the short arm. However, this method has specific requirements on the polarization direction of the incident light. Once the environment of the transmission fiber changes, which causes the polarization direction of the incident light to change, the polarization control unit also needs to modulate correspondingly, so that the polarization direction of the incident light is always aligned to the non-cut optical axis of the unequal-arm interferometer to keep the interference effect stable, otherwise, the intensity of the light pulse entering the interferometer changes, which affects the final interference effect. Obviously, this can greatly increase the complexity of the stability control (e.g., a complicated feedback mechanism may need to be designed) and the design difficulty of the control system.

Fig. 2 shows another receiving end structure in the prior art for solving the above-mentioned problems. As shown in the figure, in this scheme, a polarization maintaining beam splitter PBS is introduced, and two sets of decoding modules are provided, which are respectively connected to two output terminals of the PBS. Similar to the scheme shown in fig. 1, each decoding module comprises a polarization-maintaining unequal arm interferometer, in which the fast or slow axis of the fiber (for light pulses) is cut off, and two detectors. With this arrangement, the polarization directions of the two outputs of the PBS are perpendicular to each other, one fast/slow axis cutoff polarization maintaining unequal arm interferometer is connected to each, and the polarization directions of the two output ports outputs are aligned with the non-cutoff optical axes of the respective polarization maintaining unequal arm interferometers. When the polarization direction of incident light changes, the incident light is divided into two beams determining the polarization direction, and the two beams interfere with each other. The ratio of the light intensity of the two beams depends on the polarization direction of the incident light, and the total light intensity is kept stable. Thus, the interference effect of the two unequal arm interferometers remains generally stable. However, this method is complicated in structure and high in cost, and is not favorable for use and maintenance.

Disclosure of Invention

Aiming at the defects of the solutions in the prior art, the invention provides a phase coding system and a receiving end thereof, compared with the existing solutions, the phase coding system has a simpler structure and a better and more stable environment, can provide better interference contrast and reduce the decoding error rate, and is particularly suitable for quantum key distribution.

One aspect of the present invention discloses a receiving end for detecting a phase difference between two optical pulses in an optical pulse pair, the receiving end including an unequal-arm michelson interferometer 2 and a detection module 3. Wherein the unequal-arm michelson interferometer 2 comprises a beam splitting element 21, a long arm 22, a short arm 23, a first reflecting element 24 and a second reflecting element 25. The beam splitting unit 24 may be arranged to split the light pulse into two parts, and one of the two parts propagates along the long arm 22 towards the first reflecting unit 24; the other of the two parts propagates along the short arm 23 towards the second reflecting element 25. The optical path difference between the long arm 22 and the short arm 23 is set to be the same as the time interval between the two light pulses. The first reflection unit 24 and the second reflection unit 25 may be arranged to deflect the polarization direction of the light pulse portions by 90 degrees while reflecting the light pulse portions. And, the detection module 3 may be configured to detect the interference result output by the unequal arm interferometer 2.

Preferably, the phase difference between the two polarized light pulses in the pair of light pulses is one of 0, pi/2, pi, 3 pi/2.

The receiving end of the present invention may further include an optical transmission unit 1. The optical transmission unit 3 may have a first port, a second port and a third port and is arranged such that the pair of optical pulses propagates via the first port and the second port towards the interferometer 2 and one of the interference results output by the interferometer 2 is transmitted via the second port and the third port towards the detection module 3. Preferably, the optical transmission unit 1 is a circulator.

Preferably, the first and second reflecting units 24 and 25 may be faraday mirrors.

Preferably, the detection module 3 may comprise a single photon detector.

The receiving end of the present invention may further include a phase shift unit 4 for compensating for a phase shift of the optical pulse occurring during the propagation.

The receiving end of the present invention may further include a basis vector selecting unit 5 for selecting X basis vector coding and Y basis vector coding.

Preferably, the basis vector selecting unit 5 may be provided on at least one of the long arm 22 and the short arm 23 of the unequal-arm interferometer 2.

Preferably, the basis vector selecting unit 5 may be disposed outside the unequal arm interferometer 2.

In another aspect of the invention, a phase encoding system is disclosed, comprising a transmitting end and the receiving end of the invention, the transmitting end being arranged to output the optical pulse pair for the receiving end. The transmitting end may include a light source, an unequal arm interferometer, and a phase modulation unit. The unequal arm interferometer may be arranged to split a light pulse output by the light source into two light pulses adjacent in time. And the phase modulation unit is arranged to phase modulate at least one of the two optical pulses to form a phase difference therebetween.

A further aspect of the invention discloses a phase encoding system which may comprise a transmitting end and a receiving end of the invention, the transmitting end being arranged to output the optical pulse pairs for the receiving end. The transmitting end comprises a light source and a phase modulation unit. The light source is arranged to generate two light pulses adjacent in time within a time period based on pulse injection locking and inter-modulation. And the phase modulation unit is arranged to phase modulate at least one of the two optical pulses to form a phase difference therebetween.

Drawings

The present invention will be described in further detail with reference to the accompanying drawings and specific embodiments.

Fig. 1 shows a receiving end structure for a phase encoding scheme in the prior art;

fig. 2 shows another receiving end structure for a phase encoding scheme in the prior art;

FIG. 3 illustrates one embodiment of a phase encoding system according to the present invention; and

fig. 4 shows another embodiment of a phase encoding system according to the invention.

Detailed Description

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following examples are provided by way of illustration in order to fully convey the spirit of the invention to those skilled in the art to which the invention pertains. Accordingly, the present invention is not limited to the embodiments disclosed herein.

Fig. 3 shows an embodiment of a phase encoding system according to the invention. For the purpose of illustration, the working principle of the receiving end of the present invention will be explained first, but those skilled in the art know that the receiving end is not only applicable to the phase encoding scheme, but can be used in any application where the phase difference between two pulses needs to be detected.

Those skilled in the art know that under the phase encoding scheme, the optical pulse pair output by the transmitting end may include two optical pulses (a front pulse and a rear pulse), and the relative phase difference between the front and rear pulses may be one of 0, pi/2, pi, 3 pi/2 as a phase encoded value. The receiving end receives the coded optical pulse pair and performs phase decoding on the coded optical pulse pair, so as to determine a code value carried by the optical pulse pair.

As shown in fig. 3, the receiving end may include an optical transmission element 1, an unequal-arm michelson interferometer 2, and a detection module 3.

When the encoded optical pulse pair reaches the receiving end, it enters the unequal arm michelson interferometer 2 via the optical transmission element 1.

The unequal-arm michelson interferometer 2 may include a beam splitting element 21, a long arm 22, a short arm 23, a first reflecting element 24, and a second reflecting element 25.

The first light pulse of the pair of light pulses reaches the beam splitting unit 21 first and is split into two parts, i.e. the first slow light and the first fast light, which propagate along the long arm 22 and the short arm 23, respectively. As a preferred example, the beam splitting unit 21 may be a beam splitter BS.

In the interferometer 2, the front slow light propagates along the long arm 22 toward the first reflecting unit 24. In the present invention, the reflection units 24 and 25 may be disposed to deflect the polarization direction of the incident light by 90 degrees while reflecting the incident light. Therefore, after the front slow light is reflected by the first reflection unit 24, the polarization direction is changed by 90 degrees and returns to the beam splitting unit 21 in the reverse direction. Similarly, the front fast light will travel along the short arm 23 toward the second reflecting unit 25, and after being reflected by the second reflecting unit 25, the polarization direction changes by 90 degrees and returns to the beam splitting unit 21 in the reverse direction. As a preferred example, the reflecting unit may be a faraday mirror.

Thereafter, the rear light pulse of the pair of light pulses reaches the beam splitting unit 21 and is split into two parts, i.e., rear fast light and rear slow light, which propagate along the long arm 22 and the short arm 23, respectively. In the interferometer 2, the rear slow light travels along the long arm 22 toward the first reflection unit 24, is reflected by the first reflection unit 24, changes in polarization direction by 90 degrees, and returns to the beam splitting unit 21 in the reverse direction. The latter fast light will propagate along the short arm 23 towards the second reflecting unit 25, after reflection by the second reflecting unit 25 the polarization direction changes by 90 degrees and returns in the reverse direction to the beam splitting unit 21.

In the present invention, the optical path difference between the long arm 22 and the short arm 23 may be set to correspond to the time interval between the front and rear pulses, thereby enabling the front slow light and the rear fast light to return to the beam splitting unit 21 at the same time and to interfere. Therefore, the beam splitting unit 21 will output two paths of interference results outwards.

As a preferred example, the detection module may include a first detection unit 3-1 and a second detection unit 3-2. The first detection unit 3-1 and the second detection unit 3-2 may be arranged for detecting two interference results, respectively. Preferably, the detection unit may be a single photon detector.

By means of the unequal-arm interference structure, when an optical pulse passes through the long and short arms, polarization changes can be caused due to the difference of the long and short arm optical fibers, the reflection unit (such as a Faraday mirror) can rotate the polarization direction of the optical pulse by 90 degrees and reflect the optical pulse, so that the optical pulse returns along the original path of the long and short arms, the optical pulse is transmitted back and forth in the long and short arm optical fibers due to the fact that the effect of the optical pulse is equal to per-unit conversion, and the forward path and the reverse path show completely complementary optical fiber fast and slow axis paths, so that phase disturbance caused by the fact that the optical pulse is different in fast and slow axis paths due to polarization disturbance during unidirectional transmission can be counteracted, the stability of phase difference between two paths of the optical pulse which finally generate interference is ensured, and high interference contrast is realized.

As a preferred example, the optical transmission unit 1 may have three ports and be arranged such that the encoded optical pulse pair propagates towards the beam splitting unit 21 via the first port and the second port, and the one-way interference result output by the beam splitting unit 21 propagates towards the first detection unit 3-1 via the second port and the third port. Still further, the optical transmission unit may be a circulator, particularly preferably a fiber optic circulator.

The receiving end may further include a phase shift unit 4 for compensating for a phase shift of the optical pulse occurring during propagation. As shown in fig. 3, as an example, the phase shift unit 4 may be provided on the short arm of the interferometer 2. The phase shifting unit 4 may also be arranged at other suitable positions, for example on the long arm of the interferometer 2, as is known to the person skilled in the art.

The receiving end may further include a basis vector selecting unit 5. In the phase encoding scheme, the basis vector selection unit 5 may preferably be a phase modulator that enables selection of the X-basis vector or the Y-basis vector by applying a certain phase difference (e.g., 0 or pi/2) between two optical pulses in an optical pulse pair.

As a preferred example, the phase modulator 5 may be arranged in the interferometer 2, for example on the long arm 22 or the short arm 23. For example, as shown in fig. 3, the phase modulator 5 is disposed on the long arm 22, and the operation principle of the basis vector selecting unit 5 will be explained below by taking this as an example.

The phase modulator 5 randomly modulates the phase of 0 or pi/2 to the front slow light. If the phase modulator 5 modulates the phase 0 (corresponding to the selected X basis vector), if the phase difference between the pair of optical pulses output from the transmitting end is 0, the interference result is that the first detecting unit 3-1 has an output and the second detecting unit 3-2 has no output. When the phase difference between the optical pulse pairs output by the transmitting end is pi, the interference result is that the first detection unit 3-1 has no output, and the second detection unit 3-2 has output. Therefore, the receiving end can correspondingly realize the decoding process of the X-base vector phase coding at the moment. Accordingly, when the transmitting end performs phase encoding under the X basis vector, when the phase difference of the phase modulator 5 to the previous slow light modulation is ± pi/2, the probabilities of generating outputs at the first and second detection units are the same.

If the phase modulator 5 modulates the pi/2 phase (corresponding to the selection of the Y basis vector), when the phase difference between the pair of optical pulses output from the transmitting end is pi/2, the interference result is that the first detecting unit 3-1 has no output and the second detecting unit 3-2 has output. When the phase difference between the optical pulse pairs output by the transmitting end is 3 pi/2 (or-pi/2), the interference result is that the first detection unit 3-1 has output, and the second detection unit 3-2 has no output. Therefore, the receiving end can correspondingly realize the decoding process of the Y-basis vector phase coding at the moment. Accordingly, when the transmitting end performs phase encoding under the Y basis vector, when the phase difference of the phase modulator 5 to the previous slow light modulation is 0 or pi, the probabilities of generating outputs at the first and second detection units are the same.

In the preferred example in which the basis vector selection unit 5 is provided in the anisometric arm interferometer 2, the optical pulse is reflected after being rotated by 90 ° in polarization direction by the faraday rotator mirror after passing through the phase modulator 5 in the forward direction, and then returns through the phase modulator again in the reverse direction. Since the forward and reverse optical pulses are polarized vertically, the light components corresponding to the two optical axes entering the phase modulator in the forward and reverse directions are exchanged, which has the advantage of canceling out the modulation error caused by the difference in the phase modulation effect of the optical pulses on the two optical axes of the phase modulator 5. Furthermore, under such an arrangement, the basis vector selecting unit 5 can be easily used for light pulses having an arbitrary polarization direction, i.e., without making any requirement on the polarization direction of the light pulses.

As another preferred example, the basis vector selecting unit 5 may be disposed outside the interferometer 2, for example, in front of the optical transmission element 1, as shown in fig. 4. In this example, the optical pulse passes through the phase modulator 5 only in one direction, which causes less optical loss to the optical pulse, and the modulation signal of the modulator 5 does not need to complete the high-low level conversion within the time when the optical pulse forward leaves and reversely arrives at the phase modulator, so that the requirement on the modulation signal of the phase modulator 5 can be reduced, and the requirement on the corresponding electronic system can be correspondingly reduced. However, the phase modulator 5 at this time needs to have the same phase modulation effect for incident light of any polarization state. If this requirement is not satisfied, if the polarization direction of the incident optical pulse changes due to a change in the fiber environment, the phase modulation effect changes, and the error rate increases.

Referring again to fig. 3 and 4, the phase encoding system of the present invention is shown, respectively. As shown, the phase encoding system includes a transmitting end, a receiving end, and a transmission optical path for connecting the transmitting end and the receiving end.

The transmitting end is used for outputting optical pulse pairs with phase encoding. As shown in fig. 3 or 4, as an example, the transmitting end may include a light source, an unequal arm interferometer, and a phase modulation unit.

In the present invention, the light source may be a laser, such as a fiber laser. Light pulses output by the light source enter the unequal arm interferometer. The unequal-arm interferometer may include a long arm, a short arm, and two beam splitting units. Preferably, since the light pulse output by the light source is polarized light, the long arm and the short arm can be implemented by polarization-maintaining fibers, thereby eliminating polarization changes caused by fiber discrimination of the long and short arms. The optical pulse is divided into two beams by one of the two beam splitting units, and the two beams are respectively converged to the other of the two beam splitting units through the polarization maintaining optical fiber to be combined. Because the optical paths provided by the long arm and the short arm are different, the optical pulse output by the light source is divided into two optical pulses which are temporally divided into front and back, so that an optical pulse pair is formed. The phase modulation unit is configured to phase-modulate the optical pulses (for example, the previous optical pulse or the next optical pulse) in the optical pulse pair, thereby randomly modulating a phase difference of one of 0, pi/2, pi, and 3 pi/2 between the previous and next optical pulses in the optical pulse pair as a phase-encoded value.

Although the phase modulation unit is shown in fig. 3 and 4 as being disposed outside the unequal arm interferometer, those skilled in the art will readily appreciate that in the transmitting end, the phase modulation unit may also be disposed within the unequal arm interferometer, for example, on the long arm and/or the short arm of the interferometer.

Those skilled in the art will readily understand that in the phase encoding system of the present invention, the transmitting end is not limited to the structure shown in fig. 3 and 4, and any encoding structure capable of outputting a phase-encoded optical pulse pair may be adopted. As another preferred example, the transmitting end may include a light source based on an injection locking manner and a phase modulation unit. In such an encoding mechanism, the light source may be arranged to generate a pair of light pulses at a time based on the principle of pulse injection locking and the internal modulation process, and then the pair of light pulses is phase-modulated by the phase encoding unit to achieve the corresponding phase encoding, whereby the arrangement of the unequal arm interferometer may be omitted.

Based on the foregoing description, it can be seen that the phase encoding system proposed by the present invention can solve the problem of the decrease in the receiving count rate caused by the polarization disturbance due to the difference between the long and short arms of the unequal-arm interferometer and the polarization disturbance of the transmission fiber in the prior art for phase encoding in a simple and effective manner, thereby implementing a simplified, stable and low-error-rate phase encoding system.

The above description is not intended to limit the present invention, and the present invention is not limited to the above examples. Those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

Claims (12)

1. A receiving end for detecting a phase difference between two optical pulses in a pair of optical pulses, the receiving end comprising an unequal arm interferometer (2) and a detection module (3), characterized by:

the unequal-arm interferometer (2) comprises a beam splitting unit (21), a long arm (22), a short arm (23), a first reflecting unit (24) and a second reflecting unit (25), wherein,

the beam splitting unit (24) is arranged to split the light pulse into two parts, and one of the two parts propagates along the long arm (22) towards the first reflecting unit (24); the other of said two portions propagating along said short arm (23) towards said second reflecting element (25);

the optical path difference between the long arm (22) and the short arm (23) is set to be the same as the time interval between the two light pulses;

the first reflection unit (24) and the second reflection unit (25) are arranged to deflect the polarization direction of the light pulse portion by 90 degrees while reflecting the light pulse portion; and is

The detection module (3) is arranged for detecting the interference result output by the unequal arm interferometer (2).

2. The receiving end of claim 1, wherein a phase difference between two polarized light pulses in the pair of light pulses is one of 0, pi/2, pi, 3 pi/2.

3. The receiving end according to claim 1, further comprising an optical transmission unit (1), the optical transmission unit (3) having a first port, a second port and a third port and being arranged such that the pair of optical pulses propagates via the first port and the second port towards the unequal arm interferometer (2) and one of the interference results output by the unequal arm interferometer (2) is transmitted via the second port and the third port towards the detection module (3).

4. The receiving end according to claim 3, wherein the optical transmission unit (1) is a circulator.

5. The receiving end according to claim 1, wherein the first reflecting unit (24) and the second reflecting unit (25) are faraday mirrors.

6. The receiving end of claim 1, wherein the detection module (3) comprises a single photon detector.

7. The receiving end according to claim 1, further comprising a phase shift unit (4) for compensating a phase drift of the optical pulse occurring during propagation.

8. The receiving end according to claim 1 or 2, further comprising a basis vector selection unit (5) for selecting an X basis vector code and a Y basis vector code.

9. The receiving end according to claim 8, wherein the basis vector selection unit (5) is arranged on at least one of the long arm (22) and the short arm (23) of the unequal-arm interferometer (2).

10. The receiving end according to claim 8, wherein the basis vector selection unit (5) is arranged outside the unequal-arm interferometer (2).

11. A phase encoding system comprising a transmitting end and a receiving end according to any one of claims 1 to 10, the transmitting end being arranged to output the optical pulse pairs for the receiving end, wherein:

the transmitting end comprises a light source, an unequal arm interferometer and a phase modulation unit;

the unequal arm interferometer is arranged to split a light pulse output by the light source into two light pulses adjacent in time; and

the phase modulation unit is arranged to phase modulate at least one of the two optical pulses to form a phase difference therebetween.

12. A phase encoding system comprising a transmitting end and a receiving end according to any one of claims 1 to 10, the transmitting end being arranged to output the optical pulse pairs for the receiving end, wherein:

the transmitting end comprises a light source and a phase modulation unit;

the light source is arranged to generate two light pulses adjacent in time within a time period based on pulse injection locking and inter-modulation; and

the phase modulation unit is arranged to phase modulate at least one of the two optical pulses to form a phase difference therebetween.

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