JPH0591048A - Optical transmitter - Google Patents

Abstract

PURPOSE:To obtain an optical communication equipment stable against temperature in which coherent optical communication employing a polarized wave scramble at a high speed data rate is facilitated and the modulation condition is simply changed. CONSTITUTION:The optical communication equipment applies polarized wave self modulation of an external resonance laser and the resonator is provided with an optical path length modulation means to apply FSK modulation so that its rate of change is sufficient with respect to the length of the resonator. The optical ring resonator forms a resonator structure by three full reflection mirrors 11A and one partial reflection mirror 11Aa. An electrooptic crystal refraction rate modulator 1 receiving a modulation signal is provided to one optical axis between the adjacent mirrors 11A and a semiconductor laser medium 3, a lens 5, a polarized wave conversion element 7A and a wavelength conversion element 9A are provided to the other optical axis.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical transmitter for coherent optical transmission.

[0002]

2. Description of the Related Art Conventionally, when performing coherent optical transmission, it is known that the reception sensitivity is significantly deteriorated when the polarization of the signal light and the polarization of the local oscillation light do not match at the time of reception. However, the light propagating through the ordinary optical fiber changes its polarization from time to time due to pressure, temperature change, vibration, etc. applied to the optical fiber. Several methods have been conventionally proposed to deal with this polarization.

For example, active polarization control is performed by using a fiber squeezer or the like so as to match the polarizations of the signal light and the local oscillation light, and the polarization is received by dividing the signal light and the local oscillation light into orthogonal polarizations. Diversity, polarization scrambling in which polarization of signal light is switched and transmitted at a speed higher than the bit rate on the transmission side, and polarization maintaining fiber in which the optical fiber itself does not easily cause polarization fluctuation are known.

Among them, polarization scrambling is essentially 3
Although there is a deterioration in the receiving sensitivity of dB, no special measures against polarization are required on the receiving side, which is attractive in a system having a large number of receivers with respect to a small number of transmitters.

Up to now, two types of methods for applying polarization modulation to signal light have been roughly classified and reported. One is a method of arranging a high-speed polarization modulator using the electro-optic effect at the output end of signal light (for example, Electronics Letters Vol.23 (1987) No.10).
pp513-514 TGHodgkinson and others ”POLARISATION-INSENSITI
VE HETERODYNE DETECTION USING POLARISATION SCRAMB
LING ”: However, here, instead of the polarization modulator, a phase modulator, a wavelength element, and a multiplexer / demultiplexer are used.),
The other is to pass signal light subjected to FSK (Frequency Shift Keying) through a birefringent medium, and the polarization is switched according to the optical frequency of the data depending on the wavelength (frequency) dependence. Method (eg OFC ^, 89
WG4 "Polarisation-insensitive 500-Mbit / s FSK
transmission over 153 km by passive polarisation s
witching ”R.noe and others).

The latter type of polarization scrambling will be described with reference to FIG.

First, a semiconductor laser 4 as a beam source
1. On the optical axis of the laser light emitted from the semiconductor laser 41, a lens 5a for converting the laser light into parallel light, an isolator 43 for suppressing return light to the semiconductor laser 41, and a constant polarization fiber. Lenses 5b for condensing light are respectively arranged on 147a.

This semiconductor laser 41 can change the injection current I to perform FSK modulation. That is, "0" of the digital signal is converted into the optical frequency f ₁ , and "1" of the digital signal is converted into the optical frequency f ₂ .

Next, the orthogonal components incident on the polarization-maintaining fiber 147a are defined as E _xi and E _yi, and the emitted light is similarly defined as E _x0 and E _y0.
And At this time, in the constant polarization fiber 147a, the x component and the y component respectively propagate with different propagation constants β _x and β _y .

[0010] The length of the polarization maintaining fiber 147a is L, respectively emitted light _{E x0, E y0 E x0 =} E xi e A (1) where _{A = -jβ x L E y0 =} E yi e B ( 2) However, B = -jβ _y L.

On the other hand, the propagation constants β _x and β _y can be expressed by the following equations using the equivalent refractive indices n _x and n _y of the constant polarization fiber 137a.

Β _x = 2πn _x / λ = 2πn _x f / c (3) β _y = 2πn _y / λ = 2πn _y f / c (4) where f is the optical frequency and c is the speed of light in a vacuum. is there.

Now, if the incidence conditions are set so that E _xi = E _yi , the following equation is obtained from equations (1) to (4).

[0014]

[Equation 1]

Further, when f = f ₁ , f ₁ L ( _ny−
If n _x ) / c = m (m is an integer), the equation (5) becomes the following equation (6).

E _x0 = E _y0 (6) When f = f ₂ , f ₁ L ( _ny− n _x ) / c = m + 1
If it is / 2, the equation (5) becomes the following equation (7).

E _x0 = −E _y0 (7) If FSK modulation is performed under the above conditions, the polarization at f ₁ and the polarization at f ₂ are orthogonal at the output end of the constant polarization fiber 147 a. I understand that Generally, the orthogonal condition of polarization at the output end is given by the following equation (8).

(F ₁ −f ₂ ) L ( _ny −n _x ) / c = 1/2 (8) If modulation is performed under such a modulation condition, the signal light of f ₁ and f ₂ of The polarization of the signal light is orthogonal to each other, and at least _one of the signal f _{1 and} the signal f ₂ can be received without using the polarization diversity receiver. Therefore, the configuration of the optical receiver can be greatly simplified.

Further, as described above, since it suffices to satisfy the expression (8) as the modulation condition, in this method (f ₁
When the value of −f ₂ ) is changed, the fiber length L is also changed. That is, the constant polarization fibers 14 of different lengths
7a, so that the constant polarization fiber 147
Try to change the length of a.

[0020]

However, in the former example of polarization scrambling, the method of arranging a high-speed polarization modulator using the electro-optic effect at the output end of the signal light is such that the polarization scrambling is at a bit rate. Modulators faster than the bit rate are needed because they have no effect unless they are done at a faster rate, which results in the speed of the modulator limiting the bit rate.

Further, the latter method of switching the polarization depending on the optical frequency of the data due to the wavelength dependence of the birefringent medium is only applicable to binary FSK modulation, and moreover, the data "1" Since the difference between the optical frequency corresponding to "" and the optical frequency corresponding to "0" is fixed by hardware, the degree of freedom of the bit rate and the modulation index is lost.

Further, the method of changing the length of the constant polarization fiber in order to satisfy the equation (8) as the modulation condition requires several hundreds of meters of the constant polarization fiber. The transmitter has a large size, and this constant polarization fiber 14
Since the thermal expansion coefficient of the primary coating resin is large in 7a, a temperature change varies the value of (n _y -n _x), the system is may become unstable.

As described above, in the conventional optical transmitter, it is difficult to increase the bit rate or the modulation condition cannot be changed easily, the device is large and unstable with respect to temperature. There was a problem.

A first object of the present invention is to facilitate coherent optical communication using polarization scrambling at a high data rate.

A second object of the present invention is to provide a polarization scrambler which can easily change the modulation condition, is small and is stable against temperature.

[0026]

In order to solve the above problems, an optical transmitter according to the first invention of the present application provides a semiconductor laser medium and a polarization of laser light emitted from the semiconductor laser medium in an optical resonator. The gist of the present invention is to carry out modulation by polarization self-modulation, having polarization conversion means for conversion and optical path length modulation means for modulating the optical path length in the optical resonator.

According to a second aspect of the present invention, a semiconductor laser medium is provided in an optical ring resonator formed by a pair of corner cubes, and polarization conversion means for converting the polarization of the laser light emitted from the semiconductor laser medium. And an optical path length modulation means for modulating the optical path length in the optical ring resonator to perform modulation by polarization self-modulation.

In the third invention of the present application, a semiconductor laser medium and a polarization conversion means for converting the polarization of the laser light emitted from the semiconductor laser medium are arranged in an external resonator constituted by a plurality of mirrors. The present invention is characterized in that it further comprises an optical path length modulation means for modulating the optical path length in the external optical resonator, and performs modulation by polarization self-modulation.

A fourth aspect of the present invention is characterized in that the optical path length modulating means in the optical transmitter according to the first to third aspects modulates the optical path length in the optical resonator by a current flowing through the semiconductor laser medium. ..

A fifth aspect of the present invention is characterized in that wavelength selecting means having wavelength selectivity for the light is arranged on the optical path of the optical resonator in the optical transmitting apparatus according to the first to fourth aspects.

The sixth invention of the present application is that the incident beam is incident on the first beam.
Means for splitting into the second beam and the second beam, orthogonal means for making the polarization of the first beam and the polarization of the second beam split by this splitting means orthogonal, and the splitting means An optical path length difference giving means for giving an optical path length difference to the optical path length of the first beam and an optical path length of the second beam; and a first beam given an optical path length difference through the optical path length difference giving means. The gist is to have a combining means for combining the second beam.

Further, in the seventh invention of the present application, splitting means for splitting the incident beam into a first beam and a second beam, a polarization of the first beam split by the splitting means, and a second beam. An orthogonal means for making the polarization of the beam orthogonal to each other, an optical path length difference giving means for giving an optical path length difference between the optical path lengths of the first beam and the second beam divided by the dividing means, and the optical path length. An optical path length difference changing means for changing the optical path length difference given by the difference giving means to an arbitrary value; a first beam and a second beam to which the optical path length difference is given through the optical path length difference giving means.
The gist of the present invention is to have a combining means for combining the above-mentioned beams.

[0033]

The optical transmitter of the first invention of the present application is an application of the phenomenon of polarization self-modulation of an external resonator laser, and the optical path length modulation means is provided in the optical resonator to provide FSK. The rate of change with respect to the cavity length is set to be sufficiently long so that modulation can be applied.

In the second invention of the present application, an optical ring resonator composed of a pair of corner cubes is used to make it compact and easy to adjust. At that time, an optical path for modulating the optical path length in the optical ring resonator is used. The long modulation means is disposed inside or outside the optical ring resonator to perform polarization self-modulation.

The third invention of the present application is miniaturized by using an external resonator composed of a plurality of mirrors. At that time, an optical path length modulating means for modulating the optical path length in the external optical resonator is used. It is arranged inside or outside the resonator to perform modulation by polarization self-modulation.

According to a fourth aspect of the present invention, the optical path length in the optical resonator is modulated by the electric current flowing in the semiconductor laser medium to constitute the optical path length modulating means in the optical transmitting apparatus according to any one of claims 1 to 3. ..

Further, in the fifth invention of the present application, means having wavelength selectivity is arranged at an appropriate position in the optical resonator so that no resonance occurs in the optical resonator except for a desired longitudinal mode and laser oscillation does not occur. To do.

In the optical transmitter of the sixth invention of the present application,
An optical circuit is used which splits the light of the semiconductor laser into a first beam and a second beam, gives a large optical path length to the second beam, and then performs polarization synthesis. In other words, instead of obtaining the equation (8) L a _{(n y -n x) (L} y -L x)
Is a way to get.

That is, in general, the optical path length is the physical length L.
Is multiplied by the refractive index n, while L ( _ny −
n _x) is the original (Ln _y -Ln _x), the difference between the y-polarization and x-polarized optical path length of the y-polarized wave in the refractive index is different from the birefringent material in wave Ln _y and x polarization of the optical path length Ln _x Therefore, if a physical length difference (L _y −L _x ) is given to the y polarized wave and the x polarized wave without using a birefringent substance, the optical path length difference L ( _ny −n _x ). Accordingly, the optical circuit using a fixed polarization fiber (n _y -n _x) it was necessary to set the number 100m to for small L, the number since the present invention is obtained directly (L _y -L _x) cm Can be realized in the size of.

Further, in addition to the sixth aspect of the present invention, the seventh aspect of the present invention is provided with an optical path length difference changing means for changing the optical path length difference given by the optical path length difference giving means to an arbitrary value for easy modulation. The conditions can be changed.

[0041]

First, the polarization self-modulation of an external cavity laser used in the present invention will be briefly described. This polarization self-modulation of an external cavity laser was recently reported as a method of millimeter wave generation (IEEE Photonics Technolog
y Letters, Vol.2 (1990), No.7, pp467-469, WHLoh et al. ”
Polarization Self-Modulation at MultigigahertzFreq
uencies in an External-Cavity Semiconductor Lase
r ”).

That is, as shown in FIG. 17, the polarization conversion element 112 (1/2 wavelength plate or the like) is arranged in the ring resonator together with the semiconductor laser medium 3 having the antireflection coating. In the earliest state of laser oscillation, when a certain linearly polarized light emitted from the semiconductor laser medium 3 propagates in the optical resonator, it is orthogonal to the first polarized wave in the polarization conversion element 112. Converted to polarized waves. This converted light is
When the light goes around the optical resonator and returns to the inside of the semiconductor laser medium 3, the semiconductor laser medium 3 oscillates with the light of the same polarization as the incident light instead of the light that has been oscillating until then. become.

Therefore, the laser light extracted from the optical resonator is such that the polarization is switched every time the light makes a round in the optical resonator (in the above-mentioned literature, the polarized light is emitted to the exit of the optical resonator). It states that it is possible to generate microwaves and millimeter waves by arranging a detection device such as a child and a photodiode). That is, since the switching time of this polarization changes according to the optical path length in the resonator (hereinafter also simply referred to as the resonator length), the resonator length must be increased in order to speed up the polarization switching time. It is clear that this can be achieved by shortening (in the above-mentioned document, a glass surface having a slight reflection in the optical resonator is defined as a distance between the glass surface and the laser medium is an odd number of resonator lengths). It is stated that the speed can also be increased by inserting so that it becomes 1).

As a method of shortening this resonator length, in the literature, the ring resonator may be made small as it is, but if it is desired to perform polarization switching at a higher speed, as shown in FIG. 18, the Fabry-Perot is used. Quarter wave plate 13 in the resonator
The cavity length can be made shorter by adopting the structure in which 9 is placed (Applied Physics Letters Vol.56 (1990) No.25,26W.
H. Loh et al. "High-frequency polarization self-modula
tion and chaotic phenomena in external cavity semi
conductor lasers "), the ultimate form is 100G if integrated lasers and polarization conversion elements are integrated on the same substrate.
It states that it is possible to increase the switching speed up to about Hz.

Therefore, if the device is used as a polarization scramble light source for coherent optical communication, the bit rate is not limited because high speed modulation cannot be performed as described in the conventional example. That is, in normal polarization scrambling, it is said that the switch rate of polarization needs to be double the minimum bit rate, but at present, the fastest bit rate of optical communication is about 20 Gbps (higher speeds cannot be achieved. It is considered impossible due to the problem of electronic circuits.) Even then, the rate of the polarization switch is 4
Since 0 GHz is sufficient, it can be implemented by using the above-mentioned device.

Next, a method of applying data modulation to light will be described. There are roughly three types of modulation methods for coherent optical communication. That is, A that modulates the amplitude of light
SK (Amplitude Shift Keying), FSK that modulates the optical frequency, and PSK (Phas that modulates the phase of the light)
e Shift Keying).

Of the three systems, only ASK and FSK can be used when polarization scrambling is applied to the transmitted light. For example, it is very easy to apply ASK modulation using the above device. An external modulator may be placed outside the optical resonator, and the current injected into the laser medium may be O
It is also possible to perform direct modulation by turning off N and OFF.
However, it is known that ASK generally has lower reception sensitivity than FSK, and as described above, the reception sensitivity is not so good in the case of performing polarization scrambling which essentially has a deterioration in reception sensitivity of 3 dB. Using ASK is not a good idea.

Next, regarding the FSK, the normal FSK
In the case of a semiconductor laser device formed of only a semiconductor laser medium, the injection current to the semiconductor laser device is changed to change the refractive index of almost the entire resonator or most of the optical resonator. Be seen. However, in the case of a laser device having an external resonator, the ratio of the semiconductor laser medium length to the total resonator length becomes extremely small, and modulation in the laser medium is sufficient to perform FSK. The frequency cannot be modulated. Furthermore, in this external resonator structure, the longitudinal mode of the laser becomes multimode oscillation, and there is a possibility that it cannot be used as it is as a light source for coherent optical communication.

Therefore, in the present invention, a refractive index modulation device that occupies most of the resonator length in the polarization self-modulation laser optical resonator or a device that spatially changes the optical resonator length is used. , The optical path length of the optical resonator can be changed sufficiently to perform FSK, or the portion of the external resonator is made as short as possible to the extent that it can be compared with the length of the semiconductor laser medium. Make direct modulation possible. Further, by arranging a wavelength selective element at an appropriate position in the optical resonator, no resonance occurs in the optical resonator other than the desired longitudinal mode and laser oscillation does not occur.

Embodiments of the present invention will be specifically described below with reference to the drawings. The optical ring resonator shown in FIG. 1 shows the simplest configuration of the embodiment according to the present invention. First, the configuration will be described. The optical ring resonator shown in FIG. 1 has a resonator structure with three total reflection mirrors 11A and one partial reflection mirror 11Aa. Further, of the four optical axes set between the adjacent mirrors 11A, the electro-optic crystal refractive index modulator 1 is disposed on one side of a pair of parallel optical axes, and the semiconductor laser medium 3 and the lens are disposed on the other side. 5, a polarization conversion element 7A and a wavelength selection element 9A are arranged.

Further, in this optical ring resonator, in order to obtain a sufficient speed of the polarization switch, a pair of parallel optical axis lengths in which elements are not arranged do not contact elements arranged in other optical axes. Set as short as possible.

The electro-optic crystal refractive index modulating element 1 as the optical path length modulating means is composed of LiNbO ₃ or the like, and is arranged so as to fill the space between the mirrors 11A.

The lenses 5 and 5 arranged at both ends of the semiconductor laser medium 3 collimate the light in the ring resonator into parallel rays. In addition, the wavelength selection element 9A and each resonator mirror 1
Non-reflective coating is applied to both ends of the elements other than 1A. Each of the four resonator mirrors 11A and 11Aa has no polarization dependence in reflectance, and three of the mirrors
1A has a reflectance as close to 100% as possible, and the remaining one mirror 11Aa has a reflectance less than 100% so that light can be extracted to the outside.

As the polarization conversion element 7A, a ½ wavelength plate, a 90 ° Faraday rotator, etc. can be considered. Furthermore, an electrode may be attached to the electro-optic crystal, or a coil may be wound around the magnetic material of the Faraday rotator so as to have characteristics corresponding to those by electrically tuning. When a half-wave plate (or its equivalent) is used, its axis should be oriented so that polarization conversion will occur correctly.

When an element such as a Fabry-Perot etalon that cannot be subjected to antireflection coating is used as the wavelength selection element 9A, the angle is slightly inclined so that the reflected light does not return to the semiconductor laser medium 3, or , The distance from the semiconductor laser medium 3 is set to be an odd fraction of the entire cavity length. This is because, as described above, if there is a reflective surface in the optical resonator, counterclockwise and clockwise coupling occurs there, and the switching speed of polarization changes at the position of the reflective surface. Also, although it is possible to apply a non-reflective coating such as a dielectric multilayer filter, those that have essentially reflective characteristics inside should be placed at the same position as in the case of the Fabry-Perot etalon. Is. In selecting a wavelength, an element whose principle does not depend on reflection inside the element, for example, a traveling wave type element such as a Mach-Zehnder type element is coated with an antireflection coating.

Further, as shown in FIGS. 2 and 3, the refractive index modulator using the electro-optic crystals 13A, 13B or the like has a bulk crystal provided with an electrode 15A, or a waveguide 19 is provided on a crystal substrate to provide an electrode 15B. It is possible to add something with
Pay attention to the crystal cutting-out direction, how to apply the bias voltage, the amplitude of the voltage at the time of modulation, etc., and operate so that there is no polarization dependence.

The optical ring resonator shown in FIG. 4 has four mirrors 11A, 11A and 11 of the optical ring resonator shown in FIG.
The corner cubes 21A are replaced with A and 11Aa for easy alignment. At this time, the reflectance is less than 100% on only one surface, as in the above-described device.

Also, in this way, the corner cube 21A
, The data bit rate is several hundred Mbp
At a low speed up to about s, the electro-optic crystal refractive index modulation element 1 is removed as shown in FIG. 5, and instead, the piezo element 23A is attached to one corner cube 21B,
It is also possible to vibrate the piezo element 23A by the data signal source 25 and change the optical path length, thereby changing the oscillation frequency and performing FSK modulation. in this case,
The size can be further reduced by moving the polarization conversion element 7A and the wavelength selection element 9A to the space created by removing the electro-optic crystal refractive index modulation element 1.

As described above, when it is desired to further increase the speed of the polarization switch, a normal external resonator and a 1/4 wavelength plate may be used instead of the ring resonator as described above. In this case as well, FSK modulation can be similarly applied. An example thereof is shown in FIG.

In the normal external resonator shown in FIG. 6, a quarter wavelength plate, a 45 ° Farade rotator, or their electrical equivalents are used as the polarization conversion element 7A. Inside the two end faces of the semiconductor laser medium 3,
That is, the inner surface of the optical resonator is coated with a non-reflective coating, and the opposite surface has a reflectance of less than 100% so that light can be extracted.

The external resonator mirror 11B has a reflectance of 100%, but it is located at the end of the elements arranged inside the optical resonator, like the normal external resonator shown in FIG. , In FIG. 7, the outside of the polarization conversion element 7B is 1
A high reflection coat close to 00% may be used.

The wavelength selection element 9 may be individually provided as in the case of the ring resonator described above, but in this case, attention must be paid to the insertion angle and position of the element as described above. Therefore, in FIG.
By adopting a DBR (Distributed Bragg Reflector) structure, the wavelength selection element 9 is included in the semiconductor laser medium 3A, and it is not necessary to pay attention to the insertion position. At this time, the semiconductor laser medium 3A is provided with antireflection coating on both ends. Further, the semiconductor laser medium 3A may have a multi-electrode structure including a plurality of electrodes 15C as shown in FIG. Further, as shown in FIG. 10, a method of using the resonator mirror 11B shown in FIG. 8 as the diffraction grating 33 can be considered.

As a method of performing FSK modulation, as shown in FIG. 11, as in the case of the ring resonator, the piezoelectric resonator 23B is attached to the external resonator mirror 11C and the data signal source 25 causes the piezoelectric resonator 23B to be connected. May be vibrated in the resonator length direction.

Further, as shown in FIGS. 12 and 13, the lens is integrated with other elements by forming the lens structure 35 on the end surface of the laser medium on the inner side of the optical resonator to form a polarization conversion element. Even if the ratio of the length of the laser medium to the total cavity length becomes large, it will flow to the laser medium if the length is comparable to the semiconductor laser medium or much smaller than that. It is also possible to directly apply FSK modulation by changing the amount of current. For example, the quarter-wave plate can be made as long as the length of the laser medium.

Furthermore, as shown in FIG. 14, if the polarization conversion element 7C is used as a waveguide and integrated with the laser medium 29D, the lens between the polarization conversion element 7C and the laser medium 29D can also be removed. Therefore, the total resonator length is shortened, and the efficiency of direct FSK modulation can be increased.

As described above, according to the above embodiments,
By arranging an element in which the optical path length is sufficiently changed with respect to the total resonator length in the optical resonator of the polarization self-modulating laser, F
By placing an element having SK modulation and wavelength selectivity inside the optical resonator, it is possible to prevent multimode oscillation in the longitudinal mode from occurring, and it is possible to realize an ultra-high-speed polarization switch and It is possible to provide a polarization scramble light source for coherent optical communication that does not depend on the rate or the FSK modulation index.

Next, another embodiment of the optical transmitter according to the second aspect of the present invention will be described. FIG. 15 shows an embodiment of the optical transmitter according to the present invention. In FIG. 15, the semiconductor laser 4
1, the lens 5, the isolator 43, the polarization beam splitter 45, the lens 5 and the optical fiber 47 are the semiconductor laser 4
A linear cube is arranged on the optical axis of the laser beam emitted from the laser beam No. 1, and a corner cube 21A is arranged at a position orthogonal to the optical axis of the polarization beam splitter 45.

The light emitted from the semiconductor laser 41 is converted into parallel light by the lens 5a, and the optical isolator 4
Pass 3. Since the optical isolator 43 utilizes the Faraday effect, the one using one Faraday rotator has its polarization plane rotated by 45 °. Therefore, if the semiconductor laser 41 is oscillating in the TE mode, the polarization plane rotates by 45 ° after passing through the isolator 43, and the light is split into two in the polarization beam splitter (PBS) 45A. One of the two halves of the light is returned by the corner cube 21A and enters the PBS 45B, where it is polarized and combined with the other one of the beams. The polarized and combined light is condensed on the optical fiber 47 by the lens 5b.

In the present invention, the orthogonal condition of polarization at the output end is given by equation (9) instead of equation (8).

2 (f ₁ −f ₂ ) K / c = 1/2 (9) Here, K is the distance between the PBS and the corner cube 21A.

For example, if f ₁ -f ₂ = 2 [GHz], then K = 3.75 cm, an extremely small optical transmitter can be constructed, and the entire optical system is assembled using a material having a small coefficient of thermal expansion as a substrate. As a result, the temperature can be stabilized. Then, by moving the corner cube 21A and changing K, it is possible to easily satisfy the expression (9).

It is needless to say that two mirrors may be used instead of the corner cube 21A, but the corner cube 21A can relax the angular accuracy when fixing and the accuracy of the mechanism for moving the corner cube 21A. Leads to relaxation.

When the isolator 43, the Faraday element has a two-stage structure, or the like, a λ / 2 wave plate may be inserted and rotated by 45 ° after passing through the isolator 43. Alternatively, as shown in FIG. 16, a beam splitter 49 of equal distribution may be used instead of the PBS (4), and a λ / 2 wavelength plate 41 may be inserted in one optical path to make the polarization planes of the two beams orthogonal to each other.

Further, some or all of the above optical circuits may be integrated on the same substrate, and this optical transmitter may be realized by an integrated optical circuit.

[0075]

As described above, the optical transmitter of the present invention is the first to easily perform coherent optical communication using polarization scrambling at a high data rate,
Secondly, when the polarization scrambling is performed, the modulation condition can be easily changed, and the small size and the excellent temperature stability can be obtained.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing a configuration of the present invention.

FIG. 2 is an example of a polarization conversion element used in the present invention.

FIG. 3 is an example of a polarization conversion element used in the present invention.

FIG. 4 is one of the embodiments of the present invention, in which a corner cube is used as a resonator mirror.

FIG. 5 is one of the embodiments of the present invention, in which a piezo oscillator attached to a corner cube is used as an optical path length modulator.

FIG. 6 is one of the embodiments of the present invention and is an example in which an external resonator structure is adopted.

FIG. 7 is an embodiment of the present invention, which is an example in which the resonator mirror of FIG. 6 is integrated with other elements.

8 is an example of an embodiment in which the semiconductor laser medium of FIG. 6 has a multi-electrode DBR structure and is integrated with a wavelength selection element.

9 is a diagram showing the structure of the multi-electrode DBR of FIG.

10 is an example of an embodiment of the present invention, in which a diffraction grating having a function as a wavelength selection element is used instead of the total reflection mirror in FIG.

FIG. 11 is an example of an embodiment of the present invention in which a piezo oscillator integrated with a total reflection mirror is used as an optical path length modulation element.

FIG. 12 is an example of an embodiment of the present invention, in which a lens and a laser medium are integrated to shorten the entire resonator length to enable direct FSK modulation.

13 is an example showing the structure of a laser medium integrated with the lens of FIG.

FIG. 14 is one of the embodiments of the present invention and is an example in which the whole is integrated.

FIG. 15 is a configuration diagram showing an embodiment of an optical transmitter according to the present invention.

FIG. 16 is a configuration diagram showing another embodiment according to the present invention.

FIG. 17 is an explanatory diagram showing a structure of a conventional polarization self-modulating laser of a ring resonator type.

FIG. 18 is an explanatory view showing the structure of a conventional external cavity type polarization self-modulating laser.

FIG. 19 is a configuration diagram of a conventional optical transmitter.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Electro-optic crystal refractive index modulator 3 Semiconductor laser medium 5 Lens 7 Polarization conversion element 9 Wavelength selection element 11 Total reflection mirror 11a Partial reflection mirror 13 Electro-optic crystal 15 Electrode 17 Power supply 19 Waveguide 21 Corner cube 23 Piezo oscillator 25 Data signal source 27 Highly reflective film 29 Other electrode DBR structure Semiconductor laser medium 31 Bragg diffraction grating 33 Diffraction grating 35 Lens medium 41 Semiconductor laser 43 Isolator 45 Polarization beam splitter 47 Optical fiber 49 Beam splitter 51 λ / 2 wavelength plate

Claims (7)

[Claims] 1. A semiconductor laser medium in an optical resonator, a polarization conversion means for converting the polarization of laser light emitted from the semiconductor laser medium, and an optical path length modulation means for modulating the optical path length in the optical resonator. And an optical transmitter that performs modulation by polarization self-modulation. 2. A semiconductor laser medium and a polarization conversion means for converting the polarization of laser light emitted from the semiconductor laser medium are arranged in an optical ring resonator constituted by a pair of corner cubes, An optical transmission device further comprising: an optical path length modulation unit that modulates an optical path length in the optical ring resonator to perform polarization self-modulation. 3. A semiconductor laser medium and a polarization conversion means for converting the polarization of laser light emitted from the semiconductor laser medium are arranged in an external resonator constituted by a plurality of mirrors, and further, An optical transmission device, comprising: an optical path length modulation means for modulating an optical path length in an external optical resonator to perform modulation by polarization self-modulation. 4. The optical transmitter according to claim 1, wherein the optical path length modulation means modulates the optical path length in the optical resonator by a current flowing through the semiconductor laser medium. 5. The optical transmitter according to claim 1, wherein wavelength selecting means having wavelength selectivity with respect to the light is arranged on an optical path of the optical resonator. 6. A dividing means for dividing an incident beam into a first beam and a second beam, and a polarization of the first beam and a polarization of the second beam divided by the dividing means are orthogonal to each other. The orthogonal means for biasing, the optical path length of the first beam split by the splitting means, and the second
Optical path length difference imparting means for imparting an optical path length difference to the optical path length of the beam, and a multiplexer for multiplexing the first beam and the second beam to which the optical path length difference is imparted via the optical path length difference imparting means. An optical transmitter comprising: 7. A dividing means for dividing an incident beam into a first beam and a second beam, and a polarization of the first beam and a polarization of the second beam divided by the dividing means are orthogonal to each other. The orthogonal means for biasing, the optical path length of the first beam split by the splitting means, and the second
An optical path length difference giving means for giving an optical path length difference to the optical path length of the beam, an optical path length difference changing means for changing the optical path length difference given by the optical path length difference giving means to an arbitrary value, and the optical path length difference giving means An optical transmission device, comprising: a combining unit that combines the first beam and the second beam that have been provided with an optical path length difference via the.