Optical transmitter and method for generating polarization multiplexed transmission light

Abstract

Provided are: an optical transmitter capable of attenuating, with a simple configuration, only one polarization of polarization multiplexed light; and a method for generating polarization multiplexed transmission light. An optical transmitter (10) comprises: a polarization multiplexing modulator (11); and a polarizer (12) that receives input of polarization multiplexed light LDP output by the polarization multiplexing modulator (11) and that generates polarization multiplexed transmission light LDPTX in which one polarization (V) of the polarization multiplexed light LDP is attenuated in accordance with a predetermined extinction ratio RE.

Description

Optical transmitter and method for generating polarization-multiplexed transmitted light

[0001] The present invention relates to an optical transmitter in an optical communication system, and more particularly to a technology for generating polarization multiplexed transmission light.

[0002] In recent years, various optical transmission technologies have been proposed to cope with the ever-increasing communication traffic. Polarization multiplexed optical transmission has attracted attention in particular because it enables twice the amount of information to be transmitted using the same wavelength band. For example, Patent Document 1 discloses an example of a polarization multiplexed transmitter.

[0003] In the field of optical communications, quantum key distribution (QKD) systems are being actively researched and put into practical use as a way to achieve high confidentiality in transmission paths. Continuous-variable QKD, in particular, is attracting attention because it can use ordinary photodiodes at room temperature instead of highly sensitive photon detectors, making it possible to measure the quantum noise limit (Patent Document 2).

[0004] According to Patent Document 2, a sender (Alice) terminal splits laser light into reference light (hereinafter referred to as LO (local oscillator) light) and signal light using a beam splitter, randomly phase-modulates the signal light, and then attenuates it to generate weak signal light, and transmits the generated weak signal light and LO light to a receiver (Bob) terminal. The receiver terminal randomly phase-modulates the arriving LO light, and combines the phase-modulated LO light with the arriving weak signal light to extract phase information of the signal light.

[0005] JP 2012-119759 A JP 2000-101570 A

[0006] However, applying polarization multiplexing transmission to continuous-quantum QKD requires attenuating only one polarization of the polarization multiplexed signal at Alice's terminal, which requires an optical circuit that separates the polarization multiplexed light output from the polarization multiplexing IQ modulator into two orthogonal polarizations, attenuates one polarization, and finally combines them, complicating the circuit configuration following the polarization multiplexing IQ modulator.

[0007] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an optical transmitter that can attenuate only one polarization of polarization multiplexed light with a simple configuration, and a method for generating polarization multiplexed transmission light.

[0008] According to a first aspect of the present invention, an optical transmitter includes a polarization multiplexing modulator and a polarizer that receives polarization multiplexed light output from the polarization multiplexing modulator and generates polarization multiplexed transmission light in which one polarization of the polarization multiplexed light is attenuated according to a predetermined extinction ratio. According to a second aspect of the present invention, a method for generating polarization multiplexed transmission light in an optical transmitter includes the steps of: a polarization multiplexing modulator that generates polarization multiplexed light; and a polarizer that generates polarization multiplexed transmission light in which one polarization of the polarization multiplexed light is attenuated according to a predetermined extinction ratio.

[0009] According to the present invention, it is possible to attenuate only one of the polarizations of polarization multiplexed light with a simple configuration.

[0010] Fig. 1 is a block diagram showing a schematic configuration of an optical transmitter according to the present disclosure. Fig. 2 is a schematic diagram for explaining the function of a polarizer in an optical transmitter according to the present disclosure. Fig. 3 is a schematic diagram for explaining a QKD key generation process. Fig. 4 is a block diagram showing a first example of a continuous quantity QKD system to which an optical transmitter according to the present disclosure is applied. Fig. 5 is a block diagram showing a second example of a continuous quantity QKD system to which an optical transmitter according to the present disclosure is applied. Fig. 6 is a block diagram showing a third example of a continuous quantity QKD system to which an optical transmitter according to the present disclosure is applied.

[0011] <Overview of the Embodiments> The inventors of the present invention focused on the extinction ratio of a polarizer (also called a polarizing plate or polarizer) and discovered that the blocked polarized light can be used as a weak signal light. In other words, by simply passing polarization multiplexed light through a polarizer, the extinction ratio of the polarizer can be used to set the desired optical intensity ratio between the transmitted linearly polarized light and the blocked linearly polarized light. Therefore, simply adding a polarizer after a polarization multiplexing modulator can generate polarization multiplexed light in which only specific polarization components are attenuated, making it easy to realize a small and lightweight optical transmitter. Hereinafter, embodiments and examples of the present invention will be described in detail with reference to the drawings.

[0012] 1, an optical transmitter 10 according to this embodiment includes a polarization multiplexing modulator 11, a polarizer 12, and a laser light source 13. The polarization multiplexing modulator 11 receives laser light from the laser light source 13 and multiplexes orthogonal vertical polarization (V polarization) and horizontal polarization (H polarization) into polarization multiplexed light L. ＤＰ is output to the polarizer 12.

[0013] The polarization multiplexing modulator 11 includes a branching device 101, modulators 102 and 103, and a polarization multiplexing device 104. Laser light from a laser light source 13 is branched into two by the branching device 101, one branched light being input to the modulator 102 and the other branched light being input to the modulator 103. The modulator 102 modulates one branched light in accordance with a modulation signal M1 and outputs a first transmitted light to the polarization multiplexing device 104. The modulator 103 modulates the other branched light in accordance with a modulation signal M2 and outputs a second transmitted light to the polarization multiplexing device 104. The polarization multiplexing device 104 polarization-multiplexes the first transmitted light and the second transmitted light to generate polarization multiplexed light L ＤＰ and output to polarizer 12. Here, it is assumed that the first transmitted light is superimposed on a V-polarized wave, and the second transmitted light is superimposed on an H-polarized wave. Each of modulators 102 and 103 can be, for example, a nested Mach-Zehnder (MZ) modulator. Furthermore, according to this embodiment, a commercially available polarization multiplexing modulator 11, for example, an LN (lithium niobate) modulator, can be used. In other words, the optical transmitter 10 can be configured simply by connecting polarizer 12 to the output port of a single polarization multiplexing modulator 11.

[0014] The polarizer 12 has an extinction ratio R Ｅ and the polarization multiplexed light L ＤＰ The polarizer 12 is disposed in a direction to attenuate the V-polarized wave and transmit the H-polarized wave. Ｅ According to the polarization multiplexed light L ＤＰ Polarization multiplexed transmission light L in which only the V polarization is attenuated ＤＰＴＸ The polarizer 12 may be, for example, a dichroic polarizing plate made of a film having polarization properties or an in-line fiber polarizer.

[0015] Extinction ratio R of polarizer 12 Ｅ can be selected depending on the extent to which the V polarization is to be attenuated. For example, the extinction ratio RＥ = 20 to 30 dB. As will be described later, in the case of an optical transmitter in a continuous QKD system, the extinction ratio R Ｅ It is more desirable to set the extinction ratio to 25 to 30 dB. For example, if a polarizer has an extinction ratio of 30 dB, the optical intensity of the blocked polarized wave will be attenuated to 1 / 1000 of the optical intensity of the transmitted polarized wave, resulting in a weak optical power of less than 1 photon / bit. Therefore, simply providing polarizer 12 makes it easy to realize the quantum unit of the transmitting terminal Alice in continuous-quantum QKD.

[0016] The polarizer 12 may be held by a polarizer holder whose position is adjustable. The extinction ratio can be adjusted by rotating the polarizer around the optical axis using the polarizer holder or by displacing the rotational symmetry axis of the polarizer 12 from the optical axis. Also, a variable attenuator may be provided after the polarizer 12 to adjust the extinction ratio of the polarization multiplexed transmission light L ＤＰＴＸ The optical intensity of the polarized light may be attenuated to generate a V-polarized wave of a desired optical intensity (for example, the above-mentioned weak light of 1 photon / bit or less).

[0017] The following describes the function of polarizer 12, taking as an example a case where polarization multiplexing modulator 11 is a DP-IQ modulator and modulators 102 and 103 are each composed of a nested MZ modulator. Each modulator IQ-modulates laser light in accordance with a modulation signal corresponding to the transmission information, and outputs the respective transmission light.

[0018] In Fig. 2, the modulator 102 maps the transmission information to a signal point on the IQ plane in accordance with the modulation signal M1, and outputs the first transmission light to the polarization beam combiner 104. The modulator 103 modulates the laser light in accordance with the modulation signal M2, and outputs the modulated second transmission light to the polarization beam combiner 104. In this embodiment, the first transmission light IQ modulated by the transmission information becomes the signal light, and the second transmission light becomes the reference light. Note that the modulator 103 can also transmit the laser light without carrying information on it. The polarization beam combiner 104 polarization-multiplexes the first transmission light and the second transmission light by carrying them on the V polarization and the H polarization, respectively, and outputs the polarization-multiplexed light L ＤＰ is output to the polarizer 12.

[0019] The polarizer 12 has an extinction ratio R Ｅ According to the polarization multiplexed light L ＤＰOnly the V polarized light of the polarization multiplexed light L ＤＰ Polarization multiplexed transmission light L in which only the signal light is attenuated to a predetermined weak signal light. ＤＰＴＸ As described above, the polarization multiplexed light L ＤＰ A variable attenuator may be provided after the polarizer 12 so that the V-polarized wave is attenuated to a desired light intensity.

[0020] As described above, by simply connecting the polarizer 12 to the DP-IQ modulator, the polarization multiplexed transmission light L ＤＰＴＸ can be easily generated.

[0021] 2. Application Examples A typical QKD transmission system transmits random numbers, which are the basis of an encryption key, using weak light at the single photon level as a medium, thereby enabling secure key sharing between locations. By using such weak light, it is possible to quantum-mechanically guarantee that the encryption key will not be leaked, achieving a high level of confidentiality. For this reason, it is expected to be used in encrypted communications that handle highly confidential information.

[0022] Hereinafter, a system will be described in which the optical transmitter 10 described above is applied to an Alice terminal of continuous-quantum QKD that modulates and transmits the quadrature amplitude of light.

[0023] As shown in FIG. 3, continuous-quantum QKD consists of four steps: (1) weak light transmission, (2) basis matching, (3) error correction, and (4) privacy amplification.

[0024] (1) In weak optical transmission, the optical transmitter 10 described above transmits weak signal light and reference light after polarization multiplexing. In continuous QKD, encryption keys can be generated by coherent detection. Coherent detection is a technology commonly used in long-distance, high-capacity optical communications, and can therefore be realized using common optical components. Therefore, continuous QKD is expected to be less expensive than discrete QKD.

[0025] (2) Basis matching exchanges basis information between the optical transmitter and the optical receiver through a normal optical channel (classical channel), and selects received information with a matching basis. (3) Error correction is performed on the selected information (shift key) obtained by basis matching. (4) Privacy amplification is a process in which a random number is generated within the transmitter, and a new random number (final key) is generated using the generated random number to remove any information that may have leaked during key distribution. Below, we will explain the configuration and operation of continuous-quantum QKD using the optical transmitter disclosed herein.

[0026] <First Example> Figure 4 illustrates a digital coherent continuous-quantum QKD system. An optical transmitter 30 according to the present disclosure includes a DP-IQ modulator 31, a polarizer 32, a laser light source 33, a variable optical attenuator (VOA) 34, and a key generation control unit 35. The DP-IQ modulator 31 includes a splitter 301, IQ modulators 302 and 303, and a polarization combiner 304, and has the same basic configuration and functions as those shown in Figure 1. Laser light from the laser light source 33 is split into two beams by the splitter 301, with one branched beam input to the IQ modulator 302 and the other branched beam input to the IQ modulator 303.

[0027] The key generation control unit 35 generates a modulated signal in accordance with the key element K0 and base A (+ / x), and modulates the IQ modulators 302 and 303 of the DP-IQ modulator 31 as described above. The key element K0 is a random number sequence that serves as the element for generating the final key. The base A is also a random number sequence, but for convenience of explanation, it is expressed as + / x. Here, it is assumed that the IQ modulator 302 outputs the first transmission light (signal light) illustrated in FIG. 2, and the IQ modulator 303 outputs the second transmission light (reference light).

[0028] The signal light and reference light output from the IQ modulators 302 and 303, respectively, are polarization-multiplexed by the polarization combiner 304 as V-polarized waves and H-polarized waves, and are output to the polarizer 32. The polarizer 32 attenuates only the V-polarized waves of the signal light with an extinction ratio of 25 to 30 dB. The polarization-multiplexed transmission light L obtained in this way ＤＰＴＸ The optical intensity of the weak signal light is attenuated by the variable attenuator 34 and transmitted to the optical receiver 40 through the optical transmission line 50. The amount of attenuation by the variable attenuator 34 is set so that the optical power of the weak signal light is 1 photon / bit or less.

[0029] The optical receiver 40 includes a 90° hybrid 401 , a local laser light source 402 , a variable attenuator (VOA), a balanced detector (BD), an analog-to-digital converter (ADC) 403 , a digital signal processor (DSP) 404 , and a key generation controller 405 .

[0030] The 90° hybrid 402 is a hybrid compatible with V polarization and H polarization, and receives the polarization multiplexed received light arriving from the transmitter 30 via the optical transmission path 50 and the local laser light LO from the local laser light source 402. The optical components output from eight output optical ports are input to four balance detectors BD, two at a time, via variable attenuators VOA. Each balance detector BD detects a received signal from the two optical components, and each received signal is sampled by an ADC 403. The DSP 404 performs processing such as chromatic dispersion compensation, polarization separation, and equalization on the received signal sampled by the ADC 403, and outputs a received signal S Ｑ－ＲＣＶ to the key generation control unit 405. The polarization separation process in the DSP 404 is to separate into desired polarization components.

[0031] As mentioned above, the receiving method of interfering weak signal light with local laser light LO is called coherent detection. In coherent detection, the optical amplification effect of the signal light can be obtained by interfering local light with high optical power with the signal light. Therefore, even if the signal light power is weak, less than 1 photon / bit, it can be detected using a general photodetector.

[0032] In particular, the intradyne method is becoming mainstream in digital coherent optical receivers. The intradyne method allows a certain degree of offset rather than matching the frequencies of the weak signal light and the local laser light LO. The effect of the beat frequency due to the frequency offset can be compensated for by the DSP 404, which has the advantage of eliminating the need for highly accurate wavelength control of the local laser light source 402.

[0033] As described above, the received signal S Ｑ－ＲＣＶOnce the key generation control unit 405 of the optical receiver 40 and the key generation control unit 35 of the optical transmitter 30 exchange information through the optical transmission path 51 of the classical channel to perform the above-mentioned basis matching, error correction, and privacy amplification processes and share the final key data. Note that the polarization splitting process in the DSP 404 can also be performed optically in the 90° hybrid 402.

[0034] 5 illustrates a self-homodyne continuous quantity QKD system. The basic configuration of the optical transmitter 30 is the same as that of the first example, so the same reference numerals are used and a description thereof will be omitted.

[0035] The optical receiver 50 includes a polarization controller 501, a polarization splitter 502, a phase modulator 503, a multiplexer 504, a balance detector 505, an analog-to-digital converter (ADC) 506, and a key generation control unit 507. When the polarization multiplexed light arrives from the optical transmitter 30 through the optical transmission line 50, the polarization controller 501 aligns the polarization disturbed in the optical transmission line 50 with the polarization direction V / H of the polarization splitter 502. The polarization splitter 502 converts the polarization multiplexed light output from the polarization controller 501 into V-polarized received signal light L. Ｑ and the H-polarized received reference light L ＬＯ The received signal light L Ｑ is incident directly (or via a phase modulator, not shown) on one input port of the combiner 504. ＬＯ is phase-modulated by the phase modulator 503 with basis B and enters the other input port of the combiner 504 .

[0036] The multiplexer 504 receives the signal light L Ｑ and the phase-modulated received reference light L ＬＯ The output light is detected by a balance detector 505. The signal detected by the balance detector 505 is converted into a digital signal by an ADC 506, and a received signal S Ｑ－ＲＣＶ to the key generation control unit 507.

[0037] The key generation control unit 507 of the optical receiver 50 and the key generation control unit 35 of the optical transmitter 30 exchange information through the optical transmission path 51 of the classical channel, thereby performing the above-mentioned basis matching, error correction, and privacy amplification processes and sharing the final key data.

[0038] 6 differs from the second example in that an optical PLL (Optical Phase-Locked Loop) circuit 508 is added to the optical receiver 50. Since the other configurations are the same as those in FIG. 5, the same reference numerals are used and the description will be omitted.

[0039] In FIG. 6, an optical PLL circuit 508 receives the H-polarized reference light L separated by the polarization separator 502. ＬＯ The reference light LO is input to the phase modulator 503 and is phase-modulated in accordance with the base B, and is input to the other input port of the combiner 504. As in the second example, the combiner 504 receives the received signal light L Ｑ and a phase-modulated reference light LO are superimposed, and a reception signal S obtained from the output light is Ｑ－ＲＣＶ The final key data is shared based on the

[0040] Although the present disclosure has been described above with reference to embodiments and examples, the present disclosure is not limited to the above-described embodiments and examples. The configuration and details of the present disclosure may be modified in various ways that are understandable to those skilled in the art within the scope of the present disclosure. Furthermore, each embodiment and each example may be combined with other embodiments and examples as appropriate.

[0041] 4. Supplementary Notes Some or all of the above-described embodiments can be described as in the following supplementary notes, but are not limited to these. (Supplementary Note 1) An optical transmitter comprising: a polarization multiplexing modulator; and a polarizer that receives polarization multiplexed light output from the polarization multiplexing modulator and generates polarization multiplexed transmission light in which one polarization of the polarization multiplexed light is attenuated according to a predetermined extinction ratio. (Supplementary Note 2) The optical transmitter according to Supplementary Note 1, in which the polarizer has an extinction ratio of 20 to 30 dB. (Supplementary Note 3) The optical transmitter according to Supplementary Note 1 or 2, provided with a variable attenuator that attenuates the optical intensity of the polarization multiplexed transmission light output by the polarizer. (Supplementary Note 4) The optical transmitter according to Supplementary Note 1 or 2, in which the extinction ratio is changed by displacing the main axis of the polarizer from the optical axis of the polarization multiplexed light or by rotating the polarizer. (Supplementary Note 5) The optical transmitter according to Supplementary Note 1 or 2, wherein the polarization multiplexing modulator includes a first modulator and a second modulator that modulate laser light with transmission information and a polarization combiner, the first modulator modulates the laser light and outputs first transmission light to the polarization combiner, and the second modulator modulates the laser light and outputs second transmission light to the polarization combiner. (Supplementary Note 6) The optical transmitter according to Supplementary Note 4, wherein the first transmission light of one polarization is used as signal light and the second transmission light of the other polarization is used as reference light. (Supplementary Note 7) An optical communication system having the optical transmitter according to Supplementary Note 6 and an optical receiver connected to the optical transmitter by an optical transmission line, wherein the optical transmitter transmits the polarization multiplexed transmission light to the optical receiver through the optical transmission line, and the optical receiver comprises: a polarization separator that polarization-separates the polarization multiplexed reception light that has arrived through the optical transmission line into reception signal light and reception reference light, and an interference optical system that causes interference between the reception signal light and the reception reference light to detect a reception signal. (Supplementary Note 8) A method for generating polarization multiplexed transmission light in an optical transmitter, comprising: a polarization multiplexing modulator that generates polarization multiplexed light, and a polarizer that generates polarization multiplexed transmission light in which one polarization of the polarization multiplexed light is attenuated according to a predetermined extinction ratio. (Supplementary Note 9) The method for generating polarization multiplexed transmission light according to Supplementary Note 7, wherein the polarizer has an extinction ratio of 20 to 30 dB.(Supplementary Note 10) The method for generating polarization multiplexed transmission light according to Supplementary Note 8 or 9, in which a variable attenuator attenuates the light intensity of the polarization multiplexed transmission light output by the polarizer. (Supplementary Note 11) The method for generating polarization multiplexed transmission light according to Supplementary Note 8 or 9, in which the extinction ratio is changed by displacing a main axis of the polarizer from an optical axis of the polarization multiplexed light or by rotating the polarizer. (Supplementary Note 12) The method for generating polarization multiplexed transmission light according to Supplementary Note 8 or 9, in which a first modulator modulates laser light with transmission information and outputs first transmission light to a polarization multiplexer, a second modulator modulates the laser light with the transmission information and outputs second transmission light to the polarization multiplexer, and the polarization multiplexer outputs the polarization multiplexed light. (Supplementary Note 13) The method for generating polarization multiplexed transmission light according to Supplementary Note 12, in which the first transmission light of one polarization is used as signal light and the second transmission light of the other polarization is used as reference light. (Supplementary Note 14) An optical transmitter connected to an optical receiver by an optical transmission line in a continuous quantum key distribution system, comprising: a polarization multiplexing modulator that polarization multiplexes first transmission light and second transmission light obtained by modulating laser light with key information to generate polarization multiplexed light; and a polarizer that receives the polarization multiplexed light and generates polarization multiplexed transmission light in which one polarization of the polarization multiplexed light is attenuated according to a predetermined extinction ratio, the optical transmitter transmitting polarization multiplexed transmission light to the optical receiver through the optical transmission line, the polarization multiplexed transmission light having the first transmission light of one of the polarizations as signal light and the second transmission light of the other polarization of the polarization multiplexed light as reference light. (Supplementary Note 15) The optical transmitter according to Supplementary Note 14, wherein the polarizer has an extinction ratio of 20 to 30 dB. (Supplementary Note 16) The optical transmitter according to Supplementary Note 14 or 15, further comprising a variable attenuator that attenuates the optical intensity of the polarization multiplexed transmission light output by the polarizer. (Supplementary Note 17) The optical transmitter according to Supplementary Note 14 or 15, wherein the extinction ratio is changed by displacing a main axis of the polarizer from an optical axis of the polarization multiplexed light or by rotating the polarizer. (Supplementary Note 18) A continuous quantum key distribution system comprising the optical transmitter according to Supplementary Note 14 or 15 and the optical receiver, wherein the optical receiver polarization-separates polarization-multiplexed received light arriving through the optical transmission path into received signal light and received reference light, and causes interference between the received signal light and the received reference light to detect received information.

[0042] The present invention can be used in an optical transmitter and an optical communication system having a polarization multiplexing modulator.

[0043] 10 Optical transmitter 11 Polarization multiplexing modulator 12 Polarizer 13 Laser light source 101 Splitter 102, 103 Modulator 104 Polarization combiner

Claims

1. An optical transmitter comprising: a polarization multiplexing modulator; and a polarizer that receives the polarization multiplexed light output by the polarization multiplexing modulator and generates polarization multiplexed transmission light in which one polarization of the polarization multiplexed light is attenuated according to a predetermined extinction ratio.

2. The optical transmitter of claim 1, wherein said polarizer has an extinction ratio of 20 to 30 dB.

3. An optical transmitter according to claim 1 or 2, further comprising a variable attenuator for attenuating the optical intensity of the polarization multiplexed transmission light output from the polarizer.

4. An optical transmitter according to claim 1 or 2, wherein the extinction ratio is changed by displacing the main axis of the polarizer from the optical axis of the polarization multiplexed light or by rotating the polarizer.

5. An optical transmitter according to claim 1 or 2, wherein the polarization multiplexing modulator includes a first modulator and a second modulator that modulate laser light with transmission information, and a polarization combiner, wherein the first modulator modulates the laser light and outputs first transmission light to the polarization combiner, and the second modulator modulates the laser light and outputs second transmission light to the polarization combiner.

6. An optical transmitter according to claim 5, wherein the first transmitted light of one polarized wave is used as signal light, and the second transmitted light of the other polarized wave is used as reference light.

7. An optical communication system comprising an optical transmitter according to claim 6 and an optical receiver connected to said optical transmitter by an optical transmission line, wherein said optical transmitter transmits said polarization multiplexed transmission light to said optical receiver via said optical transmission line, and said optical receiver comprises a polarization separator which separates the polarization multiplexed reception light arriving via said optical transmission line into reception signal light and reception reference light, and an interference optical system which causes interference between said reception signal light and said reception reference light to detect a reception signal.

8. A method for generating polarization multiplexed transmission light in an optical transmitter, comprising: a polarization multiplexing modulator generating polarization multiplexed light; and a polarizer generating polarization multiplexed transmission light in which one polarization of the polarization multiplexed light is attenuated according to a predetermined extinction ratio.

9. The method for generating polarization multiplexed transmission light according to claim 8, wherein the polarizer has an extinction ratio of 20 to 30 dB.

10. A method for generating polarization multiplexed transmission light according to claim 8 or 9, wherein a variable attenuator attenuates the optical intensity of the polarization multiplexed transmission light output by the polarizer.

11. A method for generating polarization multiplexed transmission light according to claim 8 or 9, wherein the extinction ratio is changed by displacing the main axis of the polarizer from the optical axis of the polarization multiplexed light or by rotating the polarizer.

12. A method for generating polarization multiplexed transmission light as described in claim 8 or 9, wherein a first modulator modulates laser light with transmission information and outputs first transmission light to a polarization multiplexer, a second modulator modulates the laser light with the transmission information and outputs second transmission light to the polarization multiplexer, and the polarization multiplexer outputs the polarization multiplexed light.

13. A method for generating polarization multiplexed transmitted light according to claim 12, wherein the first transmitted light of one polarization is used as signal light, and the second transmitted light of the other polarization is used as reference light.

14. An optical transmitter connected to an optical receiver via an optical transmission line in a continuous quantum key distribution system, comprising: a polarization multiplexing modulator that generates polarization multiplexed light by polarization multiplexing first transmitted light and second transmitted light obtained by modulating laser light with key information; and a polarizer that receives the polarization multiplexed light and generates polarization multiplexed transmitted light in which one polarization of the polarization multiplexed light is attenuated according to a predetermined extinction ratio, and transmits polarization multiplexed transmitted light to the optical receiver via the optical transmission line, with the first transmitted light of one polarization used as signal light and the second transmitted light of the other polarization of the polarization multiplexed light used as reference light.

15. The optical transmitter of claim 14, wherein the polarizer has an extinction ratio of 20 to 30 dB.

16. An optical transmitter according to claim 14 or 15, further comprising a variable attenuator for attenuating the optical intensity of the polarization multiplexed transmission light output from the polarizer.

17. An optical transmitter according to claim 14 or 15, wherein the extinction ratio is changed by displacing the main axis of the polarizer from the optical axis of the polarization multiplexed light or by rotating the polarizer.

18. A continuous quantum key distribution system comprising the optical transmitter and optical receiver of claim 14 or 15, wherein the optical receiver polarizes and separates the polarization-multiplexed received light arriving through the optical transmission path into received signal light and received reference light, and detects received information by causing interference between the received signal light and the received reference light.

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