JPS5918878B2 - optical transmitter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides an optical transmission system for optical fiber communication in which ultra-high-speed modulation is applied, the unity of longitudinal and transverse modes is always maintained, and which is constructed in a compact size. It is related to the device.

As a conventional optical transmitter for optical fiber communication, etc., Ga
A semiconductor laser such as As, GaAlAs, or InGaAsP is used alone, and the output light is modulated by directly modulating its excitation current.

However, in semiconductor lasers, there is a time constant determined by the recombination lifetime value of injected electrons and the lifetime value of emitted photons existing in the resonator, that is, the photon lifetime, and the frequency is approximately the reciprocal of the time constant (1 GH2 ) is the maximum modulation speed limit, making higher-speed modulation impossible.
Furthermore, even when using a high-quality semiconductor laser that constantly oscillates in a single longitudinal mode, when high-speed modulation of several hundred MH2 or more is applied, the longitudinal modes become extremely multiplexed, resulting in mode competition. This can cause noise and distortion in the transmitted waveform due to the dispersion characteristics of the fiber.
As a result, it is necessary to shorten the repeater interval length of optical fiber transmission lines. Furthermore, since semiconductor lasers mainly have a parallel plane resonator configuration that utilizes cleavage planes, the stability of the transverse mode is poor, and the emitted beam shape becomes unstable or complex during high-output oscillation or high-speed modulation. There was a drawback that it became

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to solve the above-mentioned drawbacks and provide an optical transmitting device using a single-mode laser light source that is compact and capable of ultra-high-speed modulation.

In order to achieve such an objective, the present invention uses a semiconductor laser simply as a pumping light source for a solid-state laser, and takes advantage of the pure unity of longitudinal and transverse modes of a solid-state laser and the high speed of an original optical modulator. By combining a semiconductor laser, a solid-state laser, and an optical modulator, we have realized a compact, single-mode optical transmitter capable of ultra-high-speed modulation, which can be used for long-distance, ultra-high-capacity optical fiber transmission. Do it like this.

The present invention will be explained in detail below using the drawings.

An embodiment of the optical transmitter of the present invention is shown in FIG. here,
1 is a pumping GaAlAs laser with a wavelength of 0.8 μm, and 2 is an oscillation wavelength of 1.32 μm (DLiNdP4Ol2 (hereinafter L
3 is a Y3Fe5Ol2 (hereinafter referred to as YIG) optical a. Isolator, 4 is LiNbO3
The optical modulator (hereinafter referred to as LN) 5 is a single mode fiber for extracting light, and all of these are fixedly arranged on a support base 6 in this order. Note that the laser resonator 2, optical isolator 3, and optical modulator 4 will be explained in detail later using separate figures. In this embodiment, it is important that the oscillation wavelength of the excitation laser 1 is within the range of 0.800±0.005 μm.

Only at this wavelength, the LNP laser resonator 2 is 1.3
Single longitudinal mode oscillation of 2 μm can be generated. In addition, the emitted beam of the excitation laser 1 is focused by the rod lens 7A and then coupled to the LNP crystal in the rabbit laser resonator 2, and the diameter of the converged beam at this time is also 5.
It was confirmed that a thickness of approximately 0 μm or less is a necessary condition for single longitudinal mode oscillation. The wavelength of the oscillation light from the LNP laser resonator 2 is 1.32 μm, and it is linearly polarized based on the anisotropy of the LNP crystal. Also, the beam directivity angle of the oscillation beam is approximately ±1.5. Since it is small, YI
When coupling with the G isolator 3, the laser resonator 2 and the YIG isolator 3 are aligned so that the center axis of the laser resonator 2 and the center axis of the YIG isolator 3 coincide, and the LNP laser beam is coupled with the YIG isolator 3. It is sufficient to match the linear polarization direction with the direction of the incident side polarizer of the YIG isolator 3. The emitted light from the YIG isolator 3 is coupled to the LN modulator 4 via the rod lens 7B.

In this embodiment, a wide-band, low-voltage driven waveguide modulator is used as the LN modulator 4, so the entrance aperture is small, and therefore fine adjustment of the lens position is required in this lens coupling. be done. To this end, in this embodiment, components 1, 7A, 2, 3, and 4 are first fixed, and while operating components 1, 7A, 2, and 4, the rod lens 7B is (not shown) and insert it between the YIG isolator 3 and the LN modulator 4, and adjust the fine movement device to set the optimum position, that is, the maximum output light power from the LN modulator 4. , and at that position, adhesively fix the rod lens 7B to the support base 6 via the support plate 8, and finally fix the rod lens 7B
Remove it from the fine adjustment device to complete the lens installation.
The single mode optical fiber 5 may be directly adhesively fixed to the output port of the modulator 4. To operate the optical transmitter of this embodiment, first, a DC current is applied to the current supply lead wire 9 to operate the excitation laser 1, and thereby the LNP laser 2
oscillate steadily.

After that, connect 50Ω to the exit side electrode terminal 10A of the modulator 4.
A load resistor (not shown) is connected, and a modulation signal voltage is applied to the inlet side electrode terminal 10B. Due to stiffness LN
The steady light of the P laser 2 is modulated according to the modulation signal.
The modulated light is extracted from the modulator 4 via the optical fiber 5. A high-speed single mode signal light is obtained from the optical fiber 5. The YIG isolator 3 allows the modulated signal light to
This prevents the light from being reflected at the exit of the modulator 4 or the connecting end face of the optical fiber 5 and returning to the LNP laser resonator 2 again.

When the laser resonator 2 receives this returned light, it generates unstable noise. FIG. 2 shows an example of a detailed structure of the LNP laser resonator 2. As shown in FIG. LNP laser cavity is 300μm
A thickly polished HII crystal plate 12 is sandwiched between a high reflection mirror (reflectance of 99.9% or more at an oscillation wavelength of 1.32 μm) 13 and an output reflection mirror (approximately 99.7%) 14. do. Here, 15 is a spacer for adjusting the distance between the resonator mirrors 13 and 14. Each of these parts 12 to 15 is fixed within an outer case 16. 17 is the central axis of the resonator.

In this configuration, when the excitation light from the GaAlAs laser 1 enters the rabbit crystal plate 12 in a direction parallel to the central axis, the LN
The P laser resonator 2 performs steady oscillation in a single longitudinal mode. The excitation conditions at this time have already been described.
It is. Furthermore, it was confirmed that when the light emission center axis of the excitation laser 1 and the center axis of the laser resonator 2 coincide with each other, oscillation occurs in a single transverse mode. Third
The figure shows the detailed structure of the YIG isolator 3 in this embodiment.

Thickness 2.2m77! The polished YIG crystal 18 is placed in a magnetic field (1.2 k0e or more) formed by an annular samarium cobalt magnet 19 and annular yokes 20A and 20B, thereby forming a 45 Alade single rotor. Polarizers 21A tilted at 45 degrees to each other are placed on both sides of this Farad rotator.
21B is installed. 22 is each of these parts 18, 19, 2
Outer box containing 0A, 20B, 21A, 21B, 231
f:? C is its central axis.

According to the YIG isolator 3 having such a structure, it has already been described in the literature (ElectrOnics Lcter, No. 13).
As reported in Vol., pp. 171-172),
Achieves excellent isolation with extremely high transmittance (less than 1 dB loss) in one direction (for example, from left to right in Figure 3) and extremely low transmittance in the opposite direction (more than 30 dB loss). be done. Figure 4 shows the LN in this embodiment.
A detailed structure of a directional coupling waveguide type LN modulator is shown as an example of the modulator 4.

This modulator main body includes an LN substrate crystal 24 and this substrate 24.
Formed by diffusing Ti ions into 8μm wide and 5μm apart.
It consists of two waveguides 25A} and 25B, and traveling waveform electrodes 26A and 26B formed on the waveguides 25A and 25B, respectively. 27 (i) is the central axis of incidence of light incident on the waveguide 25A; 28 is a support for the modulator;

The operating principle of this modulator has already been described in the literature (IEE
EJOurnalOfQuantumElectrOn
ics9QE Volume 16, Page 754~), high-speed modulation with a band of about 3 GHz is possible with a modulation voltage (high frequency voltage applied to the terminal 10B) of about 6 V. The characteristics of the optical transmitter of this embodiment assembled with the above configuration are summarized as follows.

Next, a second embodiment of the present invention will be described in which the output power of the first embodiment is improved. In this second embodiment, a high-output excitation light source is used in place of the excitation laser 1 of the first embodiment. An example of the excitation light source is shown in FIG.
Here, 29 is a GaAlAs laser array, and the light emitted from this GaAlAs laser array 29 is transmitted through an optical fiber 30, a fiber connector 31, a fiber optic, and a rod lens 3.
2 to converge to one point on the central axis 33. 34 is a support plate for the connector 31, 35 is a hemispherical lens that supports the rod lens 32, and 36 is an outer case for the rod lens 32.

The details of this excitation light source are as explained in Japanese Patent Application No. 55-61751, and by using this excitation light source, the second embodiment has an output power several times that of the first embodiment. Obtainable. In place of the LNP laser resonator 2 used in the above embodiments, NdP,Ol4
It is also possible to use a solid-state laser resonator using a neodymium direct compound laser crystal such as , KNdP4Ol2, NdAl3(BO3)4, etc.

Furthermore, instead of the directional coupling waveguide type LN optical modulator 4,
It is also possible to use various balanced bridge waveguide type optical modulators and bulk type optical modulators using LiTaO3, KH2PO4, ZnS, GaAs crystals, etc. As explained above, according to the present invention, the semiconductor laser is simply used as an excitation light source for the solid-state laser, and the pure unity of the vertical and transverse modes of the solid-state laser is utilized.
Taking advantage of the high-speed nature of the optical modulator, it is possible to perform ultra-high-speed modulation, and the longitudinal and transverse modes are always purely single, making it possible to construct an optical transmission device suitable for optical fiber communication.

Therefore, by replacing the device of the present invention with a conventional optical transmitter using a single semiconductor laser light source, ultra-high-speed modulation becomes possible, transmission capacity increases, mode noise and transmission distortion are removed, and relaying becomes possible. The spacing distance can be significantly extended. Furthermore, the present invention does not require a large pumping light source like conventional solid-state lasers, and uses a neodymium direct compound laser crystal plate instead of using a large crystal like YAG solid-state lasers, so the device can be made small. Weight reduction can be achieved.

Furthermore, compared to conventional semiconductor laser light sources, the present invention has the advantage that the oscillation wavelength is highly stable with respect to ambient temperature, and the temperature control system installed in conventional semiconductor laser light sources can be extremely simplified. Or it can be abbreviated.

[Brief explanation of the drawing]

FIG. 1 is a perspective view showing the configuration of a first embodiment of the optical transmitter of the present invention, and FIG. 2 is a LiNdP which is a component of the first embodiment.
A cutaway perspective view showing an example of the configuration of a 4O,2 laser resonator,
FIG. 3 is a cutaway perspective view showing an example of the configuration of a Y3Fe5O, two-optical isolator, and FIG. 4 is a LiNbO
FIG. 5 is a perspective view showing an example of the configuration of a three-light modulator, and FIG. 5 is a perspective view showing an example of the configuration of a high-power pumping light source, which is a component of the second embodiment of the optical transmitter of the present invention. 1...GaAlAs laser, 2...L
iNdP4Ol2 laser resonator, 3...Y3F
e5Ol2 optical isolator, 4...LiNbO
3 optical modulator, 5... single mode optical fiber, 6
...Support stand, 7A, 7B ... Rod lens, 8 ... Support plate, 9 ... Current supply lead wire, 10A ... Outlet Side electrode terminal, 10B
...Inlet side electrode terminal, 11 ... Central axis, 12 ... LiNdP4Ol2 crystal plate, 13.
...High reflector, 14...Output reflector, 1
5...Spacer, 16...Outer box, 17
...Resonance center axis, 18...Y3Fe5
O, 2 crystal, 19... Samarium cobalt magnet, 20A, 20B... Yoke, 21A, 21B.
...Polarizer, 22...Outer box, 23...
... Central axis, 24 ... LiNbO3 substrate crystal, 25A, 25B ... Waveguide, 26A, 26B
... Electrode, 27 ... Light incidence central axis, 2
8... Support stand, 29... GaAlAs
Laser array, 30... Optical fiber, 31...
... Fiber connector, 32 ... Rod lens, 33 ... Central axis, 34 ... Connector support plate, 35 ... Hemispherical lens, 36 ...・
...outer box.

Claims (1)

[Claims] 1. A neodymium direct compound laser crystal plate, a solid-state laser resonator having laser resonator mirrors disposed on both sides of the crystal plate in the thickness direction, and in order to generate single-mode oscillation in the solid-state laser resonator. Equipped with an excitation semiconductor laser that emits a laser beam having a necessary oscillation wavelength and beam diameter, an optical isolator, and an optical modulator,
The light emission central axis of the excitation light source, the resonator central axis of the solid-state laser resonator, the light incidence central axis of the optical isolator, and the light incidence central axis of the optical modulator are aligned, and the excitation light source and the solid-state laser resonate. 1. An optical transmitting device characterized in that an optical isolator, an optical isolator, and an optical modulator are arranged in this order, a modulation signal is applied to the optical modulator, and an optical modulated output is extracted from the optical modulator.