JPS6124686B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to narrowband filters in the optical domain.

As the coherence of semiconductor lasers improves, applications such as optical communication systems that truly utilize the coherence of light and optical measurement are attracting attention. As one of the basic technologies for realizing these applications, ultra-stabilization of the oscillation wavelength of semiconductor lasers is particularly important. This stabilization requires a narrow band filter in the optical range that is used as a stabilization reference.

Hitherto, as this type of narrow band filter, a fold-reperot resonator and a diffraction grating have been known. However, these are generally large in size and are unsuitable for future optical integrated circuits.

An object of the present invention is to provide a narrow band filter in the optical domain that is compact and suitable for optical integrated circuits.

This invention introduces input light from a first port and splits the input light into two ports to the second and third ports.
A wavelength that is coupled to a second reflective wavelength-selective optical filter and that provides a phase difference to the reflected light of each optical filter between these optical filters and the second and third ports of the optical coupler. The above object is achieved by inserting a phase difference adding optical waveguide having dependence.

Hereinafter, this invention will be explained in detail with reference to Examples.

FIG. 1 shows a schematic configuration of a narrowband filter according to an embodiment of the present invention. In Figure 1, 1
is a 3 dB directional optical coupler, the input light is introduced from the first port 11 of this optical coupler 1, and the second,
Relative phase 0 and π/ to third ports 12, 13
2, and is branched into two with the same amplitude.

One end of the first reflective wavelength-selective optical filter 2 is coupled to the second port 12 of the optical coupler 1 via a wavelength-dependent phase difference adding optical waveguide 4.
One end of the second reflective wavelength selective optical filter 3 is coupled to the third port 13 . The reflective wavelength-selective optical filters 2 and 3 are configured, for example, by distributed constant type Bragg reflectors, and both have a second
They have reflection characteristics as shown in Figure a, and their band center wavelengths λ ₀ are the same.

The first and second reflective wavelength selective optical filters 2,
Of the light incident on 3, these optical filters 2,
The light having the wavelength that is not reflected by the 3 dB directional optical coupler 5 is incident on the first and second ports 51 and 52 of another 3 dB directional optical coupler 5 as necessary, and is output from the fourth port 54. On the other hand, the light of the wavelength reflected by the optical filters 2 and 3 passes through the second and third ports 12 and 13 of the optical coupler 1 and is multiplexed at the fourth port 1.
It is taken out to 4. The fourth port 14 of this optical coupler 1 becomes an output port as a narrow band filter. The input port as a narrowband filter is the first
This is port 11 of .

Now, from the first port 11 of the optical coupler 1, which is the input port, to the fourth port 14, which is the output port.
Considering the wavelength characteristics up to this point, when the phase difference adding optical waveguide 4 is not present, the wavelength characteristics are almost the same as the reflection characteristics of the first and second reflective wavelength selective optical filters 2 and 3, It is difficult to achieve sufficient narrowband performance.

In contrast, by providing a phase difference adding optical waveguide 4,
First and second reflective wavelength selective optical filters 2 and 3
When a predetermined phase difference is added to the reflected light of
A narrow band characteristic as shown in is obtained. The principle is as follows.

first and second reflective wavelength selective optical filters 2;
Assuming that the reflection characteristics of 3 are the same, the amplitude and phase shift of the reflected light are all the same. At this time, the complex amplitude A of the output light to the fourth port 14 of the optical coupler 1 is A=A ₀ {e ^j(2 〓 ⁺ 〓 ^/2) +e ^j 〓 ^/2 } =A ₀ e ^j1/2 (e ^j 〓 ² 〓+1) ......(1). Here, A ₀ is the relative complex amplitude, φ is the phase difference added by the phase difference adding optical waveguide 4,
When the effective refractive index of the phase difference adding optical waveguide 4 is n and the length is L, the value of φ with respect to the wavelength λ is given by φ(λ)=2π/λnL (2).

Now, as a condition for the light having the band center wavelength λ ₀ of the first and second reflective wavelength selective optical filters 2 and 3 to be extracted with the maximum amplitude to the fourth port 14 of the optical coupler 1, for λ ₀ , The value of φ is φ(λ ₀ )=πm (m=0, 1, 2......)
......(3) and the wavelengths λ ₀ ±Δλ on both sides of the center wavelength λ ₀ of the first and second reflective wavelength selective optical filters 2 and 3 and inside the bands of the optical filters 2 and 3 As a condition for the formation of a dip at λ
The values of n and L in equation (2) are selected so that the value of φ for ₀ ±Δλ becomes φ(λ ₀ ±Δλ)=π(m+1/2) (4). When such a phase difference adding optical waveguide 4 is provided, the first port 1 of the optical coupler 1 is
The wavelength characteristics between port 1 and port 14 are shown in Figure 2b.
It has interference wavelength dependence as follows. The characteristic shown in Fig. 2b has one peak at the center wavelength λ ₀ of the optical filters 2 and 3, and the dips on both sides of this peak are at wavelengths λ ₀ ±Δλ that are inside the bands of the optical filters 2 and 3. It is important that it is formed. Therefore, as a result, as a product of the characteristics shown in FIG. 2 a and b, a narrow band wavelength characteristic as shown in FIG. 2 c is obtained between the first port 11 and the second port 14 of the optical coupler 1. can be obtained.

That is, the phase difference φ added by the phase difference adding optical waveguide 4 is approximately mπ (m is an integer) at the center wavelength λ ₀ of the optical filters 2 and 3, and on both sides of the center wavelength λ 0 and at the center wavelength λ ₀ of the optical filters 2 and 3. By selecting (m+1/2)π at the wavelength λ ₀ ±Δλ inside the band of By forming a dip on both sides of the center wavelength within the band of the filters 2 and 3 while having a peak at the center wavelength of the wavelength-selective optical filters 2 and 3, it is possible to obtain It is possible to realize a narrow band filter that cannot be

The narrow band filter according to the present invention uses a small reflective wavelength-selective optical filter such as a Bragg reflector as a filter element, and is therefore suitable for use in optical integrated circuits. In particular, if a distributed constant type Bragg reflector is used, it is more advantageous for optical integrated circuits.

In addition, the wavelength characteristic in Figure 2c is wavelength λ ₀ ±2Δλ
However, after the fourth port 14 of the optical coupler 1, 1', 2', 3', 4',
As shown by 5', narrow band filters having almost the same structure as the first stage narrow band filter are arranged in cascade.
By providing the interference wavelength dependence as shown in FIG. 2, it is possible to realize the wavelength characteristic as shown in FIG. Of course, it is also possible to arrange similar filters in more stages to realize a filter with a narrower band.

Furthermore, in the above embodiment, the phase difference adding optical waveguide 4 was inserted between the second port 12 of the optical coupler 1 and the first reflective wavelength selective optical filter 2; It may be inserted between the port 13 and the second reflective wavelength-selective optical filter 3, or it may be inserted between both, and the phase difference is determined by the difference in phase shift amount between the two. may be added.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of a narrow band filter according to an embodiment of the present invention, and FIG. 2 is a diagram for explaining the operation of the embodiment. 1, 1', 5, 5'...3dB directional optical coupler,
2, 2', 3, 3'... reflective wavelength selective optical filter, 4, 4'... phase difference adding optical waveguide.

Claims (1)

[Claims] 1. Input light is introduced from the first port, and the input light is introduced from the second and third ports.
a directional optical coupler that branches into two ports, first and second reflective wavelength-selective optical filters having the same center wavelength coupled to the second and third ports of the optical coupler, respectively; a phase difference adding optical waveguide inserted into at least one of the optical filters and the second and third ports of the optical coupler and having a wavelength dependence that gives a phase difference φ to the reflected light of each optical filter; The phase difference φ is approximately mπ (m is an integer) at the center wavelength of the optical filter, and (m+1/2)π at wavelengths on both sides of the center wavelength of the optical filter and inside the band of the optical filter. by selecting a first port of the optical coupler;
The reflected light of each of the optical filters is synthesized through the second and third ports, and is extracted from the fourth port. A narrow band filter characterized by forming dips on both sides of the center wavelength within the band of the filter. 2. The narrowband filter according to claim 1, wherein the reflective wavelength-selective optical filter is composed of a distributed constant Bragg reflector.