BACKGROUND ART

The present invention relates to an optical transmitter used in an optical communication system or an optical measurement system and to an optical transmission system using the optical transmitter.

In conventional optical transmission systems, in order to reproduce a signal having been attenuated when propagating through an optical fiber, an identification and reproduction optical repeater is used which converts an optical signal into an electric signal and reproduces the optical signal after identifying a digital signal. Identification and reproduction optical repeaters have a problem of limitation of response speed of an electronic part that performs photoelectric conversion and a problem of an increase in power consumption. In consideration thereof, fiber laser amplifiers which input excitation light into an optical fiber doped with a rare-earth element to amplify signal light while retaining its light form and semiconductor laser amplifiers have emerged. These laser amplifiers are not equipped with a function for shaping deteriorated signal light waveforms. Conversely, contamination of inevitably and randomly created spontaneous emission light occurs completely independent of a signal component and S/N of signal light drops by at least 3 dB before and after amplification. A drop in S/N causes a transmission code error rate to increase during digital signal transmission and results in a decline in transmission quality.

A phase sensitive amplifier (PSA) is being considered as means for overcoming the limitations of laser amplifiers. A PSA has a function of shaping a signal light waveform or a phase signal having been deteriorated under the influence of dispersion of a transmission fiber. Spontaneous emission light having a quadrature phase that is independent of the signal can be suppressed and spontaneous emission light in phase with the signal can also be minimized. Therefore, in principle, S/N of signal light before and after amplification can be prevented from deteriorating and can be kept the same.

FIG. 1is a diagram showing a basic configuration of a PSA according to conventional art. A PSA100includes a phase sensitive amplifying unit101using optical parametric amplification, an excitation light source102, an excitation light phase control unit103, a first optical branching unit104-1, and a second optical branching unit104-2. As shown inFIG. 1, a signal light110input to the PSA100is branched into two parts by the optical branching unit104-1, with one part being incident to the phase sensitive amplifying unit101and the other part being incident to the excitation light source102. An excitation light111emitted from the excitation light source102undergoes phase regulation via the excitation light phase control unit103and is incident to the phase sensitive amplifying unit101. Based on the input signal light110and the excitation light111, the phase sensitive amplifying unit101outputs output signal light112.

The phase sensitive amplifying unit101has a characteristic of amplifying the signal light110when the phase of the signal light110and the phase of the excitation light111match each other and attenuating the signal light110when the phases are in a quadrature phase relationship of being 90 degrees out of phase. By using this characteristic to match the phases of the excitation light111and the signal light110so as to maximize amplification gain, spontaneous emission light with a quadrature phase with respect to the signal light110is not created and, at the same time, spontaneous emission light that exceeds noise contained in the signal light is not created with respect to the in-phase component. Therefore, the signal light110can be amplified without deteriorating an S/N ratio.

In order to achieve phase lock of the signal light110and the excitation light111, the excitation light phase control unit103controls the phase of the excitation light111so as to become locked with the phase of the signal light110having been branched by the first optical branching unit104-1. The excitation light phase control unit103detects a part of the output signal light112having been branched by the second optical branching unit104-2with a narrow band detector and controls the phase of the excitation light111so as to maximize amplification gain of the output signal light112.

Non-linear optical media for performing the parametric amplification described above include second-order non-linear optical materials typified by a periodic polarization-reversed LiNbO3(PPLN) waveguide and third-order non-linear optical materials typified by quartz glass fiber.

FIG. 2is a diagram showing a configuration of a PSA according to conventional art which uses a PPLN waveguide (refer to NPL1). A PSA200shown inFIG. 2includes an erbium-doped fiber laser amplifier (EDFA)201, a first second-order non-linear optical element202and a second second-order non-linear optical element204, a first optical branching unit203-1and a second optical branching unit203-2, a phase modulator205, an optical fiber expander206, a polarization maintaining fiber207, a photodetector208, and a phase-locked loop (PLL) circuit209. The first second-order non-linear optical element202includes a first free space optical system211, a first PPLN waveguide212, a second free space optical system213, and a first dichroic mirror214. The second second-order non-linear optical element204has a similar configuration and a detailed description thereof will be omitted.

Signal light250incident to the PSA200shown inFIG. 2is branched by the optical branching unit203-1with one part being incident to the second second-order non-linear optical element204. The other branched light is phase-controlled as excitation fundamental wave light251via the phase modulator205and the optical fiber expander206is incident to the EDFA201. The EDFA201sufficiently amplifies the incident excitation fundamental wave light251and causes the excitation fundamental wave light251to be incident to the first second-order non-linear optical element202. Due to the EDFA201, the first second-order non-linear optical element202is capable of obtaining sufficient power in order to obtain a non-linear optical effect from the weak excitation fundamental wave light251. In the first second-order non-linear optical element202, second harmonic light (hereinafter, SH light)252is created from the incident excitation fundamental wave light251. The created SH light252is incident to the second second-order non-linear optical element204via the polarization maintaining fiber207. At the second second-order non-linear optical element204, by performing degenerate parametric amplification with the incident signal light250and the SH light252, phase sensitive amplification is performed and output signal light253is output.

In a PSA, in order to only amplify signal light in phase with excitation light, the signal light and the excitation light must be in phase as described above or be out of phase by exactly π radians. In other words, when using a second-order non-linear optical effect, a phase ϕ2ωsof the excitation light which is a wavelength corresponding to SH light and a phase ϕωsof the signal light must satisfy a relationship expressed by expression (1) below, where n denotes an integer.
Δϕ=½(ϕ2ωs−ϕωs)=nπExpression (1)

FIG. 3is a diagram showing a relationship between a phase difference Δϕ between input signal light and excitation light and gain in the case of a PSA using a second-order non-linear optical effect according to conventional art. The diagram shows that gain (dB) represented by an ordinate is maximized when the phase difference Δϕ represented by an abscissa is −π, 0, or π.

In order to achieve phase lock between the signal light250and the excitation fundamental wave light251, first, the excitation fundamental wave light251is subjected to phase modulation by a weak pilot signal with the phase modulator205and a part of the output signal light253is branched and detected by the detector208. The pilot signal component is minimized in a phase-locked state where the phase difference Δϕ shown inFIG. 3is minimized. Therefore, feedback is performed to the optical fiber expander206using the PLL circuit209so as to minimize the pilot signal or, in other words, to maximize an amplified output signal. Phase lock between the signal light250and the excitation fundamental wave light251can be achieved by controlling the phase of the excitation fundamental wave light251with the optical fiber expander206.

As will be described below, amidst demands for increased speed and increased capacity in optical communication, an application range of PSAs are expanding in terms of corresponding modulation systems and multiplexing systems. As shown inFIG. 3, since a conventional PSA using a PPLN waveguide has a characteristic of attenuating a quadrature phase component, a conventional PSA is applicable to amplification of a modulated signal such as intensity modulation-direct detection (IMDD), binary phase shift keying (BPSK), and differential phase shift keying (DPSK) which use normal intensity modulation or binary phase modulation. However, a conventional PSA is incapable of amplification of a modulated signal such as QPSK (quadrature phase shift keying) and 8 PSK which are multilevel modulation formats.

In order to extract a carrier from a signal light in a multilevel modulation format, the higher the degree of levels of a modulated signal, the larger the number of non-linear processes that must be used. In such a case, it is difficult to maintain S/N of fundamental wave light generated by carrier extraction from the signal light. In addition, in a configuration of a carrier extraction method using a non-linear process a plurality of times, a WDM (wavelength division multiplexing) signal obtained by wavelength multiplexing of a plurality of signals cannot be amplified collectively.

In consideration thereof, NPL2 and NPL3 disclose configurations that enable phase sensitive amplification of a modulated signal such as QPSK to be performed by adopting a configuration based on non-degenerate parametric amplification using a pair of signal light and phase conjugate light thereof. NPL2 discloses a method using quartz glass fiber that is a third-order non-linear optical material and NPL3 discloses a method using PPLN that is a second-order non-linear optical material.

FIG. 4is a diagram showing a configuration of a PSA400which is based on non-degenerate parametric amplification according to conventional art and which uses a pair constituted by main signal light and phase conjugate light thereof as signal light. The PSA400shown inFIG. 4is constituted by an optical transmitter401and a phase sensitive amplifier420.

In order to create wavelength-multiplexed signal light405, the optical transmitter401includes a signal light source402(for example, 16 LDs 1 to 16) which outputs beams of CW light with mutually different wavelengths, a modulator403corresponding to each LD, and array waveguide gratings (AWG)404. In addition, in order to generate excitation light, the optical transmitter401includes a local oscillation light source (LO)407, an optical branch408, an EDFA409, a BPF410, a second-order non-linear optical element411for second harmonic generation (SHG), and a second-order non-linear optical element406for difference frequency generation (DFG).

In order to generate excitation light, the phase sensitive amplifier420includes a phase modulator (PM)431, an EDFA432, a BPF433, and a second-order non-linear optical element434for SHG. Furthermore, for optical parametric amplification and phase control, the phase sensitive amplifier420includes a fixed delay line424, an optical fiber expander/contractor425using a PZT piezoelectric element, a second-order non-linear optical element426for optical parametric amplification (OPA), an optical branch427, a BPF4283, a photodetector429, and a PLL circuit430.

After superimposing a data signal on each of the plurality of signal light sources402(LD1 to LD16) with the modulator403, the optical transmitter401performs wavelength multiplexing using the AWG404. SH light441is created by the second-order non-linear optical element411for SHG from fundamental wave excitation light440output from the local oscillation light source407which differs from the signal light source402. Subsequently, difference frequency light of the SH excitation light441and the main signal light group405is created using the second-order non-linear optical element406for DFG. The difference frequency light created at this point is phase conjugate light (idler light) of the main signal light405. A signal light group442constituted by a pair of a main signal light group421and an idler light group423and a part of fundamental wave excitation light443are output from the optical transmitter401. It should be noted that a frequency (a wavelength) of the fundamental wave excitation light is at a position indicated by a dotted line422with respect to the main signal light group421and the idler light group423.

The fundamental wave excitation light443output from the optical transmitter401is subjected to phase control by the phase modulator431and is incident to the EDFA432in the phase sensitive amplifier420. The EDFA432amplifies the fundamental wave excitation light output from the phase modulator431. The bandpass filter433removes noise light created by the EDFA432and only allows the fundamental wave excitation light to pass. The second-order non-linear optical element434for SHG creates SH light from the fundamental wave excitation light output from the bandpass filter433and causes the SH light to be incident to the second-order non-linear optical element426as excitation light444.

For the purpose of delay time regulation between the signal light group442and the excitation light443, the fixed delay line424delays the signal light group442output from the optical transmitter401by a prescribed time. The signal light group having passed through the fixed delay line424is subjected to phase control by the optical fiber expander/contractor425and is incident to the second-order non-linear optical element426for OPA. The second-order non-linear optical element426amplifies the signal light group and the idler light group using a parametric amplification effect.

The optical branch427branches a part of the amplified signal light group. The bandpass filter428allows only one arbitrary wavelength to pass among the signal light group and causes the passed light to be incident to the photodetector429. The photodetector429converts the incident light into an electric signal. The PLL circuit430performs feedback to the optical fiber expander/contractor425so as to maximize the electric signal detected by the photodetector429. The optical fiber expander/contractor425expands or contracts an optical fiber through which the signal light group442propagates in accordance with control output of the PLL circuit430. By controlling the phase of the signal light group442, phase lock between the signal light group421and the idler light group442and the excitation light443can be achieved.

Recent digitally-coherent optical communication systems also use polarization-multiplexed signals created by a polarization division multiplexing (PDM) technique. Since a non-linear optical medium such as a PPLN waveguide generally has polarization dependency, PSAs according to conventional art are incapable of amplifying polarization-multiplexed signals. By contrast, NPL4 and NPL5 disclose configurations which perform phase sensitive amplification on polarization-multiplexed signals with a polarization diversity configuration using two non-linear optical media to be described below with reference toFIG. 5.

FIG. 5is a diagram showing an outline of an optical transmission system according to conventional art which transmits polarization-multiplexed signal light. An optical transmission system500is constituted by an optical amplification apparatus having an optical transmitter501, an optical fiber transmission path503, and a polarization diversity configuration520. In the polarization diversity configuration520, a polarization-multiplexed signal transmitted through an optical fiber510is separated by a polarizing beam splitter (PBS)521. After performing optical amplification on the two separated polarized components by phase sensitive amplifiers522and523respectively constituted by a non-linear optical element, the polarized components are once again multiplexed by a PBS524. As will be described later, a PDM signal that is a transmitted signal of an optical transmission system that transmits polarization-multiplexed signal light is independently modulated between quadrature polarized waves by different pieces of data. In addition, unlike the configuration shown inFIG. 4in which a transmitter and a PDA are close to each other, in the optical transmission system shown inFIG. 5, the optical transmitter501and the polarization diversity configuration520are separated from each other. Therefore, excitation light is transmitted together with a PDM signal.

CITATION LIST

Non Patent Literature

NPL1: T. Umeki, O. Tadanaga, A. Takada and M. Asobe, “Phase sensitive degenerate parametric amplification using directly-bonded PPLN ridge waveguides,” Optics Express, 2011, Vol. 19, No. 7, p. 6326-6332

NPL2: M. Asobe, T. Umeki, H. Takenouchi, and Y. Miyamoto, “In-line phase-sensitive amplifier for QPSK signal using multiple QPM LiNbO3 waveguide,” In Proceedings of the OptoElectronics and Communications Conference, OECC, 2013, PDP paper PD2-3

NPL3: T. Umeki, O. Tadanaga, M. Asobe, Y. Miyamoto and H. Takenouchi., “First demonstration of high-order QAM signal amplification in PPLN-based phase sensitive amplifier,” Optics Express, February 2014, Vol. 22, No. 3, p. 2473-2482

NPL4: T. Umeki, et al., “PDM Signal Amplification Using PPLN-Based Polarization-Independent Phase-Sensitive Amplifier”, JOURNAL OF LIGHTWAVE TECHNOLOGY, 2015, Vol. 33, No. 7, pp. 1326-1332

NPL5: Masashi Abe, et al., “PDM-QPSK WDM Signal Amplification Using PPLN-Based Polarization-Independent In-Line Phase-Sensitive Amplifier,” Proceeding of European Conference on Optical Communication (ECOC) Dusseldorf 2016, paper W.4.P1.SC2.11

SUMMARY OF THE INVENTION

Technical Problem

However, with an optical transmission system which transmits polarization-multiplexed signal light according to conventional art, there are problems attributable to instability of a polarization state in an optical transmitter such as those described below. Utilizing PSA based on non-degenerate parametric amplification using a pair of main signal light and phase conjugate light thereof requires that a phase lock condition be satisfied in each quadrature polarized component with respect to all pairs of different wavelengths. With a generation method of polarization-multiplexed signal light by the optical transmitter501in an optical transmission system according to conventional art such as that shown inFIG. 5, there is a problem that an amplification operation in the phase sensitive amplifier of the polarization diversity configuration520becomes unstable.

An overview of a configuration and operations of an optical transmitter will now be provided by once again referring toFIG. 5. The optical transmitter501is constituted by two signal arms (routes) which generate different pairs of main signal light and phase conjugate light. CW light from a signal light source502of the optical transmitter501applies modulation with different data signals with respect to two quadrature polarized waves (TE polarized light and TM polarized light) using an optical modulator. Polarization-multiplexed signal light530is separated into two polarized components using a PBS504. Polarization-multiplexed signal light531of one of the separated polarized waves (TE polarized light) is rotated by 90 degrees by a polarization rotator508and shaped into a state of TM polarized light by each signal arm. Subsequently, phase conjugate light with respect to the main signal light is generated by phase sensitive amplifiers506and507respectively constituted by a non-linear optical element. After restoring the polarized wave having been rotated into TM polarized light and used to generate phase conjugate light to original TE polarized light532by a polarization rotator509, multiplexing is performed by the PBS505and a pair of a polarization-multiplexed quadrature signal and phase conjugate light or, in other words, a PDM signal533is output.

A polarized wave of the PDM signal533from the optical transmitter501is rotated inside the optical fiber510that is a transmission path. When a PDM signal534subjected to polarization rotation is input to the polarization diversity configuration520, the PDM signal534is branched into photoelectric field components projected onto two polarization axes of a first-stage PBS521. In other words, the PDM signal534is incident to non-linear optical elements522and523in a state where the two original quadrature polarized components are respectively mixed therein.

FIG. 6is a diagram conceptually illustrating a state of a polarization in an optical transmission system of PDM light according to conventional art. InFIG. 6, (a) shows two quadrature polarized waves of a PDM signal553at a point A of output of the optical transmitter501. If an abscissa represents an s polarization state, an ordinate represents a p polarization state. InFIG. 6, (b) shows a polarization state of the PDM signal534at a point B after being transmitted through the optical fiber510in which polarization rotation that cumulatively occurs over the entire optical fiber has been added to the PDM signal533at the output point A of the optical transmitter501. InFIG. 6, (c) shows an electric field component on each polarization axis of beams of signal light536and535at a point C1and a point C2after polarization and separation by the first-stage PBS521. Each polarized component of the PDM signal534at the point B is separated in a state where components projected onto the two axes of the PBS521are mixed therein.

Each of the phase sensitive amplifiers522and523which independently perform phase sensitive amplification monitors a part of output with a phase-locked configuration such as that shown inFIG. 2and applies feedback to regulate a relative phase between excitation light and a signal light group so as to maximize the output. In the polarization diversity configuration520, a PDM signal is incident in a state where two quadrature polarized components are respectively mixed therein as shown in (c) inFIG. 6. Unless a phase sensitive amplification condition is simultaneously satisfied in a stable manner in each phase sensitive amplifier with respect to the two quadrature polarized components, the PDM signal after optical amplification cannot be stabilized.

A state where two polarized components are mixed as shown in (c) inFIG. 6is in itself not a problem in the polarization diversity configuration520. However, when a polarization state of the PDM signal534received at the point B fluctuates, a PDM signal537which is an output of the phase sensitive amplifier also becomes unstable. In this case, a fluctuation of a polarization state which occurs on an optical transmission path is also not addressed. The problem here is fluctuation of the phases of the two quadrature polarized components that occur when generating a PDM signal in the optical transmitter501.

The optical transmitter501is normally constituted by a light source, an optical semiconductor element, optical parts, and a polarization maintaining fiber that connects these components to each other or the like. Furthermore, a module configuration is adopted which includes a control circuit for controlling these components, interface circuits inside and outside an apparatus, and a power supply. In such an apparatus, due to an operating state of the optical transmitter and a variation in environmental temperature, internal temperature may become non-uniformly distributed and the distribution may change constantly. In addition, depending on an environment outside the apparatus or an operating state of the apparatus, vibration from the outside may be applied to the connecting optical fibers between parts or to optical parts inside the apparatus. As described with reference toFIG. 5, in the optical transmitter501, a signal light group is respectively generated on two signal arms by separate difference frequency generators (second-order non-linear optical elements for DFG)506and507and the signal light groups are polarization-multiplexed by the PBS505. Operating states of the two signal arms may independently be affected by temperature variation, vibration, and the like. Therefore, phases of signal light, phase conjugate light, and pilot light may also vary in different modes due to the two signal arms. Furthermore, phases of excitation light supplied to the two difference frequency generators506and507may also vary in different modes. Due to individual variations in phase conditions between the two arms that generate polarization-multiplexed light, the relative phase between the two polarized waves constantly vary in a periodic or irregular manner and fluctuation may occur. In this manner, when the relative phase between the two signal light arms in the optical transmitter501contains fluctuation, the PDM signal533itself becomes unstable. The PDM signal534after being transmitted through the optical fiber510directly affects stability of phase sensitive amplification operations in the polarization diversity configuration.

Referring to (c) inFIG. 6, in the polarization diversity configuration, two polarized components are separated from PBS in a mixed state and the two mixed polarized components are input to the respective phase sensitive amplifiers. Therefore, when the relative phase of the signal light group differs between two polarized waves in an optical transmitter in which a PDM signal is formed, a phase condition for phase sensitive amplification shown inFIG. 3and Expression (1) cannot be satisfied inside one phase sensitive amplifier. After being polarization-multiplexed, since a phase change applied to the two polarized waves by the optical fiber is the same, a change in the relative phase is small. On the other hand, since a variation or fluctuation of the relative phase between the two polarized waves which is created in the optical transmitter501is more direct, there is a real-time effect between the optical transmitter501and the polarization diversity configuration520.

As described above, there is a problem in that a fluctuation in quality of a PDM signal output from the optical transmitter destabilizes output from the phase sensitive amplifiers522and523of the polarization diversity configuration520. It is difficult to generate, in a stable manner, a PDM signal constituted by a pair of a polarization-multiplexed main signal and phase conjugate light with the optical transmitter501configured according to conventional art such as that shown inFIG. 5. Therefore, there is a need for a configuration of an optical transmitter which enables a polarization diversity configuration to operate in a stable manner and which is capable of generating signal light including a pair of a polarization-multiplexed signal and phase conjugate light with a stable phase state.

The present invention has been made in consideration of the problems described above and an object thereof is to provide an optical transmitter that generates a PDM signal in which a variation in a phase between quadrature polarized waves is suppressed.

Means for Solving the Problem

In order to achieve the object described above, an invention according to claim1is an optical transmitter which generates signal light and phase conjugate light thereof, the optical transmitter including: a polarization separator to which polarization-multiplexed and modulated signal light is input and which polarization-separates signal light of a first polarized component and signal light of a second polarized component that is orthogonal to the first polarized component and outputs the signal light of the polarized components to two output ports; a loop-type phase conjugate light generator having, on a path from one to another of the two output ports, the polarization separator, a polarization rotator which rotates by 90 degrees a polarized wave of the signal light of the first polarized component, and a first second-order non-linear optical element equipped with a first optical waveguide that generates phase conjugate light of the polarization-rotated signal light and phase conjugate light of the signal light of the second polarized component; an excitation light generator which supplies first excitation light that generates the phase conjugate light of the polarization-rotated signal light to one end of the first optical waveguide on a side where the polarization rotator is present and supplies second excitation light that generates the phase conjugate light of the second polarized component to the other end of the first optical waveguide; and a phase adjuster configured to change an optical path length of one of a path of the first excitation light or a path of the second excitation light in accordance with a level of combined light of the first excitation light and the second excitation light after generating the phase conjugate light.

An invention according to claim2is the optical transmitter according to claim1, wherein the excitation light generator generates second harmonic light (SH light) from fundamental wave light, branches the SH light, and generates first SH excitation light that is the first excitation light and second SH excitation light that is the second excitation light, and the phase adjuster changes an optical path length of one of a path of the first SH excitation light or a path of the second SH excitation light. This optical transmitter corresponds to an optical transmitter according to a first embodiment.

An invention according to claim3is the optical transmitter according to claim1, wherein the excitation light generator branches fundamental wave light and, based on the branched fundamental wave light, generates first second harmonic (SH) excitation light that is the first excitation light and second SH excitation light that is the second excitation light, and the phase adjuster changes an optical path length of any of paths of the branched fundamental wave light. This optical transmitter corresponds to an optical transmitter according to a second embodiment.

An invention according to claim4is an optical transmitter which generates signal light including pilot light and phase conjugate light thereof, the optical transmitter including: a polarization separator which polarization-separates pilot light and polarization-multiplexed and modulated signal light into signal light of a first polarized component and signal light of a second polarized component that is orthogonal to the first polarized component and outputs the signal light of the polarized components to two output ports; a loop-type phase conjugate light generator having, on a path from one to another of the two output ports, the polarization separator, a polarization rotator which rotates by 90 degrees a polarized wave of the signal light of the first polarized component, and a first second-order non-linear optical element equipped with a first optical waveguide that generates phase conjugate light of the polarization-rotated signal light and phase conjugate light of the signal light of the second polarized component and performs degenerate parametric amplification in each of two directly opposite propagation directions with respect to the polarization-separated pilot light; an excitation light generator which supplies first excitation light that generates the phase conjugate light of the polarization-rotated signal light to one end of the first optical waveguide on a side where the polarization rotator is present and supplies second excitation light that generates the phase conjugate light of the second polarized component to the other end of the first optical waveguide; and a phase adjuster configured to change an optical path length of a path of the pilot light in accordance with a level of the pilot light subjected to degenerate parametric amplification in a first direction among the two propagation directions and to change an optical path length of a path of the pilot light in accordance with a level of the pilot light subjected to degenerate parametric amplification in a second direction among the two propagation directions. This optical transmitter corresponds to an optical transmitter according to a third embodiment.

An invention according to claim5is the optical transmitter according to claim4, wherein the excitation light generator generates second harmonic light (SH light) from fundamental wave light, branches the SH light, and generates first SH excitation light that is the first excitation light and second SH excitation light that is the second excitation light, and the pilot light is based on a part of the fundamental wave light.

An invention according to claim6is the optical transmitter according to any one of claim2,3, or5, wherein the excitation light generator includes one or more second second-order non-linear optical elements equipped with a second optical waveguide which generates the SH light from the fundamental wave light and generates the first SH excitation light and the second SH excitation light.

Preferably, the optical transmitter described above can further include a signal light generator which includes a plurality of light sources which output continuous wave light (CW light) and an external modulator which applies modulation to both polarized waves of the CW light from each of the plurality of light sources and which generates polarization-multiplexed and modulated signal light.

Preferably, in the optical transmitter described above, the optical waveguide included in the second-order non-linear optical element is a directly-bonded ridge waveguide, and the directly-bonded ridge waveguide can be constructed from any material of LiNbO3, KNbO3, LiTaO3, LiNb(x)Ta(1-x)O3(0≤x≤1), and KTiOPO4, or a material obtained by adding at least one material selected from a group consisting of Mg, Zn, Sc, and In as an additive to any of these materials.

Effects of the Invention

As described above, the present invention provides an optical transmitter that generates a PDM signal in which a variation in a phase between quadrature polarized waves is suppressed.

