JPS5852889A - Optical transmitter - Google Patents

Abstract

PURPOSE:To realize the small-sized device of single mode beams, which can be modulated at ultra-high speed, and to transmit an optical fiber having extra- large capacity over a long distance by simply using a semiconductor laser as the excitation light source of a solid state laser and combining the semiconductor laser, the solid state laser and a light modulator. CONSTITUTION:The GaAlAs laser 1 for excitation having 0.8mum wavelength, an LiNdP4O12 laser resonator 2 having 1.32mum oscillation wavelength, a Y3Fe5O12 optical isolator 3, an LiNbO3 light modulator 4 and a single mode optical fiber 5 for extracting beams are fixed and arranged to a supporting base 6 in this order. The laser resonator 2 can generate the single cascade mode oscillation of 1.32mum. Beams projected from the laser 1 for excitation are condensed by means of a rod lens 7A, and combined with a LNP crystal in the laser resonator 2. The LNP laser resonator 2 is formed by holding an LNP crystal plate 12 ground in 300mum thickness by a high reflector 13 and an output reflector 14. Each section 12-15 of a spacer 15 adjusting a space between the resonator mirrors 13, 14 is fixed into an outer box 16.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical transmitter for optical fiber communication, which always maintains the unity of each mode of ↓, .

As a conventional optical transmitter for optical fiber communication, etc., Ga
A single semiconductor laser such as As, GaAIAs, or InGaAsP is used, 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 ( / GHz) 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 MHz or more is applied, the longitudinal modes become extremely multiplexed, resulting in mode competition. This may cause noise, or the dispersion characteristics of the fiber may cause distortion in the transmitted waveform.
As a result, it is necessary to shorten the relay interval length of the fiber transmission line.

Furthermore, since semiconductor lasers have a parallel plane resonator configuration that mainly uses 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. It had the disadvantage of becoming

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 object, the present invention uses a semiconductor laser simply as a pumping light source for a solid-state laser, and takes advantage of the pure simplicity 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 realized a compact, single-mode optical transmitter capable of ultra-high-speed modulation, and provided it for long-distance, ultra-high-capacity optical fiber transmission. K as much as possible.

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

An embodiment of the optical transmitter of the present invention is shown in FIG.

Here, GaA/As for excitation with a wavelength of 0.1 am
v-the, λ is 32 pm to the oscillation wavelength, LiNdP40,
2 (hereinafter referred to as LNP) laser IM device, 3 is Y, F
e5O12 (hereinafter referred to as YIG) optical isolator, 4'
A FiLiNbO (hereinafter referred to as LN) optical modulator and a single mode fiber for light extraction are all arranged in this order on a support base. In addition,
Laser resonator ko, optical isolator 3 and optical modulation 'a4
1 will be explained in detail later using separate drawings.

In this example, the wavelength of the excitation laser l is Ol
It is important that it be within the range of 00±o, oos μm. Only at this wavelength, the LNP laser cavity is 7
．． Single-longitudinal mode oscillation of 32 μm can be generated.

In addition, the emitted beam of the excitation laser l is transmitted through the rod lens 7A.
After the light is focused by K, the LN inside the LNP laser resonator λ
It was confirmed that the diameter of the convergent beam at this time, which is coupled to the P crystal, must be exactly less than the diameter of SOμm, which is a necessary condition for longitudinal mode oscillation.

L, NP laser resonance wavelength of oscillation light Vi/, 3
2 μm, and is linearly polarized based on the anisotropy of the LNP crystal. Please note that the beam direction angle of the oscillation beam is approximately ±
Since i and s0 are small, when coupling with the YIG isolator 3, the laser resonator 2 and Y
It is sufficient to align the IG filter 3 and to match the linear polarization direction of the LIP laser beam with the direction of the polarizer on the incident side of the YIG filter 30.

The light emitted from the YIG isolator 3 is passed through the rod lens 7B.
LNN modulation fern coupling via. In this example, LN
Since the N modulation system employs a wide-band, low-voltage driven waveguide modulator, the entrance aperture is small, and therefore fine adjustment of the lens position is required in this lens coupling. For this purpose, in this embodiment, first, the parts /, 7
2.3 and q are fixed first, then parts/, 7A
, 2 and da, move the rod lens 7B to 3.
A shaft fine adjustment device (not shown) is temporarily fixed and inserted between the YIG isolator 3 and the LN modulator, and the fine adjustment device is adjusted to find the optimum position, that is, the output light power of the LNN modulator. Find the position where is maximum, and at that position, adhere and fix the rod lens 7B to the support base B via the support plate l, and finally remove the rod lens 7B from the fine movement device to complete the lens attachment. Incidentally, the single mode optical fiber j may be directly adhesively fixed to the output port of the modulator da.

To operate the optical transmitter of this embodiment, first, a DC current is applied to the current supply lead wire D to operate the excitation laser 1, thereby causing the LNP laser 2 to oscillate steadily. After that, the outlet of the modulator l [50
A load resistor (not shown) of Ω is connected, and the inlet side electrode terminal/17BK modulation signal voltage is applied. As a result, the steady light of the LNP laser beam is modulated in accordance with the modulation signal. The modulated light is extracted from the modulator via an optical fiber S.

A high-speed single mode signal light is obtained from the optical fiber j.

The YIG isolator 3 prevents the modulated signal light from being reflected at the exit of the modulator DA or the connecting end face of the optical fiber j and returning to the I, NPP laser oscillator 2 again. When the laser resonator λ receives this returned light, it generates unstable noise.

FIG. 2 shows an example of the detailed structure of the LNP laser co-imager. The LNP laser resonator consists of an LNP crystal plate 12 polished to a thickness of 30 pm and a high reflection mirror (oscillation wavelength 1 μm).
reflectance of 99.9% or more)/3 and output reflection rate (99.9% or more)/3
, 7%)/4I. Here, B is a spacer that adjusts the distance between the resonator mirror 13 and /l. Each of these parts /2 to /S is fixed within the outer box l≦. 17 is the central axis of the resonator. In this configuration,
When the excitation light from the GaAl Muro laser I enters the K LNP crystal plate 12 in a direction parallel to the central axis, the LNP laser resonator performs steady oscillation in a single longitudinal mode. The excitation conditions at this time have already been described. It was also confirmed that when the light emission center axis of the excitation laser 1 and the center axis of the laser resonator 1 coincide with each other, oscillation occurs in a single transverse mode.

FIG. 3 shows the detailed structure of the YIG isolator 3 in this embodiment. Thickness 2. A 4ts0 Huarade single rotor is constructed by installing it in a magnetic field (more than Hekokoe) consisting of a cowxK polished YIG crystal/ltd% annular Samusium Cobalt magnet/N and annular yokes 2DA and B. do. On both sides of this Faraday rotator, there are K 11s
Polarizers 2/A and 2/B that are tilted by .degree. are arranged.

n is each of these parts /g, /9, 2DA, 2D
B, ) An outer box containing JA and jJB, B is its central axis. According to the YIG isolator 3 having such a structure, it has already been reported in the literature (Electronics Lett.
er, Volume 13, Pages 17/-17), Tf! in one direction (e.g. from left to right in Figure 3). Extremely high Ii rate (loss/dB or less)
, such good isolation is realized that the transmittance in the reverse direction is extremely low (loss 3068 or more).

FIG. q shows the detailed structure of a directionally coupled waveguide Sugi LN modulator as an example of the LN modulator q in this embodiment. This modulator main body has a substrate crystal 24I in L and a width of 1 μm formed by diffusing Ti ions into this substrate 2G.
The two waveguides 25Aij and 2jB of tm and the traveling waveform waves &xA formed on the waveguides ``A'' and BB, respectively.
It consists of a selection of ``MuB'' and ``MuB''. The sword is the central axis of incidence of light incident on the waveguide 2jA, and d is the support of the modulator. The operating principle of this transformer has already been described in the literature (IEEE Jour
nal of Quantum Electro-ni
As stated in ``Cs, Volume QE-ty, Page 711I ~), high-speed modulation in the band J GHz is possible with a modulation voltage (higher J-voltage applied to the terminal /θB) of the order of tv. be.

The characteristics of the optical transmitter of this embodiment assembled with the above configuration are summarized as follows.

1) Wavelength 83/7μm 2) Mode Longitudinal and transverse single mode, linear polarization 5) Output power o, ty mW (power input to the extraction fiber) 4) Variable #111L pressure 5) Modulation band 3 GHz Next, a second embodiment of the present invention will be described in which the output power of the 1gl embodiment is improved.

In this second embodiment, a high-output excitation light source is used in place of the excitation laser in the first embodiment. An example of the excitation light source is shown in FIG. Here, 29 is a GaA/As laser array, and the light emitted from this GaA/NS laser array is connected to an optical fiber 30. It is made to converge to one point on the central axis 33 via the fiber connector 3 and the rod lens 3. The details are a support plate for the connector 3, a hemispherical lens that supports the 35 Halorad lens 31, and an outer case for the 1Aki rod lens 3λ. For details of this excitation light source, please refer to the patent application
3-61731, and by using this excitation light source, the second embodiment can obtain an output power several times that of the first embodiment.

Incidentally, it has been used in the above embodiments. In place of the LNP laser resonator 2, NdP50,4. KNclP40,
It is also possible to use a solid state laser cavity using a neodymium direct compound laser supply such as 2°NdAl, (BO2)4. Furthermore, instead of the directionally coupled waveguide type LN optical variable mi<<, LiTa0. , KH2PO4,ZnS
It is possible to use various balance-fridge waveguide optical modulators or bulk optical modulators using GaAs crystals or the like.

As explained above, according to the present invention, a semiconductor laser is simply used as an excitation light source for a fixed laser, and since the pure threatening nature of the longitudinal and transverse modes of a solid-state laser is utilized, the optical modulator is By taking advantage of its high speed, ultra-high-speed modulation is possible, and the longitudinal and bridge modes are always purely single, making it possible to construct an optical transmitter suitable for the original Noah transmission (4).

k 101 fU of the conventional semiconductor laser unit
, N, it becomes possible to perform supercrystal-speed modulation, increase the transmission capacity, eliminate seventh-order noise and transmission distortion, and greatly extend the distance between repeaters.

Moreover, unlike conventional solid-state lasers, the present invention does not require a large-sized pumping light source, and uses a neon compound laser crystal plate directly without using the large crystal known in YAG solid-state lasers. It is possible to realize a small and compact design.

Furthermore, compared to conventional semiconductor laser light sources, the present invention has the advantage of high stability with respect to a single wavelength around the oscillation wavelength. It can be simplified or abbreviated.

[Brief explanation of drawings]

FIG. 1 is a perspective view showing the configuration of a first embodiment of the optical transmitter of the present invention, and FIG. 1 is a LLNap which is a component of the first embodiment.
4o, a cutaway perspective view showing an example of the configuration of a laser resonator;
Figure 3 is a cutaway perspective view showing an example of the configuration of the same Y5Fe, 0.2 optical isolator, and Figure q is the same (LiNb0
．． FIG. 3 is a perspective view showing an example of the structure of a light quality detector, and FIG. 3 is a perspective view showing an example of the structure of a high-output excitation light source, which is a component of an embodiment of the optical transmitter of the present invention. / ・GaA/As v-za, Co...LiNdP40u laser resonator, 3...Y,
Fe, 0.2 optical isolator, Da...LiNb0. Optical modulator, j... single mode optical fiber, t... support stand, 71.7B-- rod lens, l... support plate,
9... Current supply lead @. □ lOm...Exit side electrode terminal, /DB...Inlet side electrode terminal, //...Center axis, 12...Li
NdP40. , knot & board, lj high reflective mirror, l
ri...output reflector, 15.spacer, 16.
・・Outer box, /7 =・Resonance center axis, /l =・Y
, Fe50,2 crystal, /9... Samarium cobalt magnet, 2f) 5) I) B... Yoke, 2/i, 2/Bi,
---Polarizer, n outer box, n middle pickle,
NoV...LiNb0. Substrate crystal, 15, EB waveguide, AA',! B...'wi pole, 1... Light incidence center axis, 2J・Support stand, 29-G
aA/As v-the array, 3θ... optical fiber, 3
/...Fino (connector, 3), rod lens, 3
3... Central axis, 3G connector support plate, 35... Hemisphere lens, 36... Outer case. Patent applicant Nippon Telegraph and Telephone Public Corporation Figure 1 Figure 2 Figure 3 Figure 5

Claims (1)

[Claims] a solid-state laser resonator having an excitation light source using a semiconductor laser, a neodymium face-edge compound laser crystal plate, and a laser resonator mirror disposed in the IIL11 direction in the thickness direction of the crystal plate;
It includes an optical isolator and an optical modulator, and includes a light output center axis of the excitation light source, a resonator center axis of the solid-state laser resonator, a light input center axis of the optical isolator, and a light input center of the optical modulator. aligning the axes, arranging the excitation light source, the solid-state laser resonator, the optical isolator, and the optical modulator in this order, and applying a modulation signal to the optical modulator to obtain an optical modulation output from the optical modulator. An optical transmitter characterized in that it can be taken out or taken out.