JPS60103316A - optical mode filter - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to an optical mode filter that filters only a specific mode among the packing modes propagating in an optical waveguide. This invention relates to a tapered mode filter that also functions as an antireflection film.

[Background of the invention]

An optical mode filter is something that acts, for example, as a normal polarizer.Of the TM waves and TE waves that propagate in an optical waveguide, an optical mode filter increases the optical absorption of the TM waves and suppresses only the TE waves. It has the function of transmitting.

Conventional optical mode filters have a structure in which a metal film is directly attached to an optical waveguide, or a structure in which a metal is attached via a dielectric buffer layer having a uniform film thickness. For example, an example of a rational sum is yamamoto t.

etc. IEEE, Journal of Qu
antulnE' e Ctr On iCS)
V O' , QB-11p729 (1975). However, these optical mode filters focus only on the attenuation of TM waves and TE waves, and do not structurally consider reflected waves between the optical mode filter and the optical waveguide section.

The end faces of optical components are usually coated with anti-reflection coating. For example, as shown in FIG. m1, light enters the optical waveguide 4 formed on the optical substrate 2 from the semiconductor laser 1 and enters the original mode filter section A. As mentioned above, the original mode filter section is A
/, , Au, etc. metal film 3 is light guided e1. It has a structure formed on Nr4.

Assuming that the optical waveguide section B is located between the semiconducting non-laser and the optical mode filter section, the electromagnetic field distribution of the propagating light is different between sections A and B, so reflected waves occur at the boundary between sections A and B. .

When this reflected wave returns to the semiconductor laser, the oscillation of the semiconductor laser becomes unstable and the oscillation spectrum fluctuates. Especially V
In the case of HF multiplexed broadband optical communication, a single mode laser (
For example, a semiconductor laser such as BH, C8P, TJS, T8, etc.) is essential, and measures to remove reflected light are essential.

The effects of reflected light on the general public are being studied in various places. - For example, as shown in FIG. 2(a), a ball lens 22 is placed in front of the semiconductor laser 21, and the radius of the ball lens is R.
and changed the Mcl between the semiconductor laser and the ball lens. By doing this, a part of the rainbow semiconductor laser light is reflected by the ball lens and the base of the light returning to the semiconductor laser changes, so it can be seen where the oscillation of the semiconductor laser becomes unstable. Figure 2 (b
) is the measured value, which is almost insensitive to the radius of the ball lens 22, and the distance d between the semiconductor laser 21 and the ball lens 220 is approximately 0.
The oscillation became unstable in the fourth state, that is, when the diameter was 400 μm or less. In this case, the amount of reflected feedback light is -65 dB or more, a
I guess. This means that the allowable reflection amount of the half-four body laser 21 is -60
It is generally said that the frequency is -70 dB or higher.

As described above, when the semiconductor laser 21 has a reflection amount of -60 dB or more, the oscillation spectrum fluctuates. Therefore, the first
As shown in the figure, the reflection at the junction between area B and the optical mode filter area must be suppressed by more than 60 dB. As shown in Figure 41, when the metal film 3 is directly attached to the optical waveguide 4, a reflection amount of about -40 dB or more occurs at the current surface of the optical waveguide 4. Furthermore, an optical mode filter has been conventionally used in which a dielectric buffer layer 5 is formed in a tapered shape as shown in FIG. 3, instead of directly attaching a metal film. However, in the case of a tapered optical mode filter, if a reflection amount of -40 dB or more occurs depending on the angle of the Oitemometer or the thickness of the buffer layer, the oscillation of the semiconductor laser becomes unstable in either case.

[Purpose of the invention]

An object of the present invention is to provide a structure of an optical mode filter that reduces reflection from the end face of the optical mode filter and ensures stabilization of oscillation of a semiconductor laser.

[Summary of the invention]

The structure of the present invention to achieve the above object includes a substrate, an optical waveguide formed on the substrate, and a parallel and tapered buffer anode having a lower refractive index on the waveguide. It consists of a seed layer of light absorption.

Since the present invention has the above configuration, the TE propagating through the optical waveguide
The propagation constants of the wave and the TM wave are real numbers, and there is no propagation loss of the βυ light. However, the propagation constant of a TE wave or a TM wave propagating through an optical waveguide complex having a medium with high light absorption, such as a metal, is a prime number.

The greater the propagation constant, the greater the reflection.

Reflection between waves with real and complex propagation constants is large. To solve this problem, the optical waveguide BK is also made of the same or similar material as the buffer layer in the gills shown in FIG. 6 may be used not only for the tapered portion but also for the parallel buffer layer portion. Ideally, it is desirable that the parallel buffer layer portion and the tapered portion change continuously and smoothly. Further, there is no particular problem if the parallel buffer layer portions have a length on the order of i length or more. Incidentally, when a buffer layer is not used in the original waveguide portion, substantially the same effect can be achieved even if the buffer layer is made into a tapered shape as shown later in FIG.

The iC will be explained in detail with reference to FIG.

The size R of the reflected wave generated when a TE wave propagating through the optical waveguide B (we will explain the TE wave here, but the same holds true for a 1゛M wave) enters a parallel metal film is approximately: is given by Here, βt is a real value, which is the propagation constant of the 1゛E wave propagating through the optical waveguide B, and βt is a complex number, which is the propagation constant of the 'rE wave propagating in parallel to the raw metal film. Here, the thickness of the buffer layer 7 is d, the thickness of the optical waveguide 4 is a, and the thickness of the substrate 2 is
Let the refractive index of the optical waveguide 4 be 02, and the refractive index of the buffer layer 7 be n3. The propagation wave number β, where n4 is the refractive index of 6), is expressed as the root of the transcendental function of (2) using the wave number k.

Here, if we increase the thickness d of Buffer No. 7 (2
) is reduced to , and the influence of the refractive index n4 disappears. In other words, the light transmission rectangle at the boundary between the optical waveguide section B and the optical mode filter section becomes almost the same.

Using the above principle, by increasing the thickness d of the burnoff layer 7, it is possible to reduce the reflection of light at the boundary between the optical waveguide section and the optical mode filter section to -60 dB or more. That is, this thickness d is given by d that satisfies equations (2), (3).

[Practice of invention]

Specific examples of the present invention will be described below. A microscope slide glass was used as the optical substrate, and the temperature was approximately 400C (DKN
It was immersed in the Oa bath solution for about 25 minutes. K I”J Os
The ions inside and the Na ions in the slide glass undergo ion conversion, and as a result, light rays are formed on the surface layer of the glass slide. When a semiconductor laser with a wavelength of 1.3 μIll was used as a light source and an optical waveguide V was used, it was confirmed that the propagating light was in a single mode. Thickness 0.45 on the optical waveguide
A tapered buffer layer having a diameter of 5402 μm was fabricated by a spuckling method. The tapered portion was fabricated by inserting a knife between the 5i02 sputter target and the optical waveguide at a height of 12 μm from the optical waveguide, and utilizing the phenomenon of the sputtered film wrapping around the lower surface of the knife.

The boundary between the fabricated taper part and the parallel Balanoa row part changed smoothly. Also, the length of the taper part is 250μm
'T: 'l>Uta. Next, use A4 as the metal film for the light absorption band, and have a length of 300mm for the tapered part and the parallel part.
Sputter deposited over μm. The wavelength of 1.3 μm is applied to the input and output ends of the optical mode filter manufactured in this way.
Anti-reflective coating applied. A wavelength of 1.3 μm (D single mode oscillation laser) was used as the semiconductor laser.

Linearly polarized light enters the optical waveguide so that the TE and T1 modes are excited at the same rate, and the instability of the optical oscillation due to reflection from the optical mode filter and the custom-made mode filter As a result, the reflection was approximately -70
It was found that the oscillation characteristics of the semiconductor laser did not change even by about dB. Also, the characteristics of the optical mode filter are shown.
It was found that the extinction ratio of E wave and TM wave is 35 dB, which shows sufficient optical mode filter characteristics.

〔Effect of the invention〕

According to this embodiment, the reflected waves from the optical mode filter fabricated in the optical waveguide can be reduced, and the effect of stabilizing the oscillation of the semiconductor laser can be improved.

[Brief explanation of the drawing]

Figure 1 shows the generation of reflected waves by combining a conventional optical mode filter and a semiconductor laser, Figure 2 shows experimental data showing the unstable region of semiconductor laser oscillation, and Figure 3
The figure is a block diagram of a tapered thin film polarizer that is an improvement over the conventional method, and FIGS. 4 and 5 are schematic diagrams of the structure of an optical mode filter as one embodiment of the present invention. l...Semiconductor laser, 2...Optical substrate, 3...Metal film, 4...Optical waveguide, 5...Tapered buffer layer, 6...Optical layer (Figure 2 (d) ) If1 interval dc4

Claims (1)

[Claims] 1. A substrate, an optical waveguide formed on the substrate, and a parallel and tapered buffer layer with a low refractive index laminated on the waveguide. An optical mode filter characterized by providing a light absorption layer. 2. In claim 1, the thickness d of the buffer nozzle is calculated by formulas (2), (3), (
5) An optical mode filter that is determined to satisfy the following.