AU603634B2

AU603634B2 - Transmitter for coherent light communication

Info

Publication number: AU603634B2
Application number: AU10188/88A
Authority: AU
Inventors: Albrecht Mozer
Original assignee: Alcatel NV
Current assignee: Alcatel Lucent NV
Priority date: 1987-01-23
Filing date: 1988-01-12
Publication date: 1990-11-22
Anticipated expiration: 2008-01-12
Also published as: AU1018888A; DE3701863A1

Description

V

L

I

6 0 363f* anlcnc>.

0 R WA AL ~st t *4 f a a o *9 t a. a a a a a (I a a a ft a a a I it I It I I a It

I

tat tI t a COMMONWEALTH OF AUSTRALIA PATENTS ACT 1952-1969 COMPLETE SPECIFICATION FOR THE INVENTION ENTITLED "TRANSMITTER FOR COHERENT LIGHT COMMUNICATION" The following statement is a full description of this invention, including the best method or performing it known to us:- This invention relates to a transmitter for coherentlight communication.

For coherent-light communication, coherent-light, narrow-band light sources are needed. In an article by T.P.

Lee, "Line-width of Single-Frequency Semiconductor Lasers for Coherent Lightwave Communications", Proceedings of the llth European Conference on Optical Communication, Venice, Italy, 1985, pp. 189 et seq., the suitability of the linewidths of conventional lasers for coherent-light communication is discussed. The lasers described there include coupled-cavity lasers as are currently widely used. Narrow line-widths are achieved, for example, by using additional Note external resonators, which are disadvantageous in that they t* have to be mounted and adjusted and that they are sensitive to ambient influences, such as temperature changes.

I The present invention has for its object to provide a transmitter for coherent-light communication which has a high power output at a narrow line-width.

S 4 According to the invention there is provided a transmitter for coherent-light communication, comprising a laser array with several parallel, phase-locked optically active regions.

The principal advantage gained by the invention are that the light source of the transmitter is extremely Snarrow-band, has a high frequency stability, and is easy to adjust.

Two embodiments of the invention will now be described with reference to the accompanying drawings, in which: Fig. I is a schematic side view of a first transmitter in accordance with the invention, which Includes a diffraction grating, Fig. 2 is a top view of the transmitter of Fig. 1; Fig. 3 is a side view of a second transmitter in accordance with the invention, which includes a prism, and Fig. 4 is a top view of the transmitter of Fig. 3.

Figs. 1. and 2 shows the first transmitter 1. according to the invention for coherent-light communication. The transmitter 2. comprises a laser array 2, a substrate 3, a I t waveguide layer 4, a modulator 5, and a diffraction grating 6. The substrate 3, the waveguide layer 4 and the 'tt diffraction grating 6 form an optical coupler 12. The waveguide layer 4{ is integrated in the substrate 3. The modulator 5 and the diffraction grating 6 are disposed on the substrate 3 in the area of the waveguide layer 4. The K laser array 2 is so positioned relative to the diffraction grating 6 that a given order of the diffracted light, here K the first order, is coupled into the waveguide layer 4. To avoid any feedback of the light from the diffraction grating 6to the laser array 2, and to reduce reflection losses, the diffraction grating 6 is provided with an antireflection coating 7i The grating constant and a favourable angle of incidence of the laser light 2 on the diffraction grating 6 follow from the wavelength lo of the laser light, from the refractive indices of the diffraction grating, the waveguide layer, and the semiconductor substrate, and from generally known optical relationships. By a suitable form of the grating (blaze angle), the light power can be transformed from the zeroth order into the first order, which results in a higher coupling efficiency. For a wavelength of lo 1300 nm, an angle of incidence of alpha 45°, and a refractive index of n 3.5, a grating constant of g 465 nm is obtained.

To modulate the laser light, use can be made of conventional modulators which modulate the amplitude, frequency, or phase using an electro-optical effect. The most common designs of such modulators are in the Mach-Zehnder interferometer and thin-film optical modulators. Depending on its design, the modulator 5 is disposed in the area of the waveguide layer 4 or forms parts of the waveguide layer i At the output 8 of the optical coupler 12, the waveguide layer 4 is provided with an antireflection coating 9. The output 8 is followed by an optical waveguide (not shown).

The laser array 2 has several lasing regions which are j arranged side by side in such close proximity that the near Sfields of the radiation produced are locked together in phase. Because of this phase lock, the laser array 2 emits extremely narrow-band radiation of wavelength lo. Such a S laser array is described in Appl. Phys. Lett. 44(2), 15 January 1984, pp. 157 to 159. The optical coupler 12 permits light from all lasing regions to be coupled into a waveguide in a simple manner, so that high output power can be obtained.

The transmitter is composed of semiconductor elements of the groups of the periodic system containing Al, Ga In, and P, As Sb. The waveguide layer 4 and the modulator Smay also be made of lithium niobate, and the antireflection coatings 7, 9 are made of silicon dioxide or silicon nitride. Electrical contacts at the laser array 2 and at the modulator 5 are not shown in the figures; they are made of metals.

Figs. 3 and 4 show a second transmitter for coherentlight communication. Like in the first embodiment, the transmitter comprises a laser array a substrate a waveguide layer a modulator and an anti-reflection coating 9' at the output which are arranged in the same manner as in the first embodiment, are made of the same materials, and perform the same functions. A prism 10 is disposed on the substrate 3' in the area of the waveguide 4' so et as to couple the light emerging from the laser array 2' into the waveguide To prevent any feedback of laser light into the laser array the prism 10 has an anti-reflection c coating 11.

The prism or diffraction grating and the waveguide layer may be made in a one piece construction of the same material. In a variation of this construction, the prism or diffraction grating may have a different refractive index from that of the waveguide layer, while still using a one piece construction.

rA

Claims

1. A transmitter for coherent-light communication, com- prising a laser array with several parallel, phase-locked optically active regions, including an optical coupler with a waveguide layer and a diffraction grating or a prism dis- posed thereon for coupling the light from the laser array into a waveguide.

2. A transmitter as claimed in claim 1, wherein the waveguide layer and the diffraction grating or the prism are of one-piece construction and are made of the same material.

3. A transmitter as claimed in claim 1, wherein the waveguide layer and the diffraction grating or the prism are of one-piece construction and are made of materials with t r different refractive indices.

4. A transmitter as claimed in claims 1 to 3, wherein a light modulator is attached to the waveguide layer. '4444* A transmitter as claimed in any one of claims 1 to 4, wherein the optical coupler contains predominantly ele- ments of the groups of the periodic table containing the el- 4 4 ao ements Al, Ga and In or P, As and Sb. f I

6. A transmitter as claimed in claim 4 or 5, wherein the optical coupler and the light modulator form an inte- grated component.

7. A transmitter as claimed in any one of claims 2 to S6, wherein the optical coupler has anti-reflection coatings at the diffraction grating or the prism and at the optical output.

8. A transmitter for coherent-light communication, sub- stantially as herein described with reference to Figs. 1 to of the accompanying drawings. DATED THIS FIFTEEENTH DAY OF AUGUST, 1990 ALCATEL N.V. I 4 4