JPH0291623A - Optical filter - Google Patents

Abstract

PURPOSE:To integrate and miniaturize an optical filter by giving the structure of a Fabry-Perot resonator provided with a multiple quantum well layer to the optical filter and changing the wavelength of transmitted light at the time of applying an electric field. CONSTITUTION:A buffer layer 21 is laminated on the upper face of a semiconductor substrate 22 of the optical filter, and an n-type electrode 23 as a first electrode is laminated on the lower face of its n<+>-type substrate. A multiple quantum well layer (MQW layer) 24 is laminated on the upper face of the n<+>-type buffer layer 21, and a cap layer 25 is laminated on the upper face of this MWQ layer 24, and a p-type electrode 26 as a second electrode is laminated on the upper face of this cap layer 25. When a reverse bias voltage is applied between the n-typ electrode and the p-type electrode 26, the equivalent refractive index is changed. Therefore, the resonance wavelength of a Fabry-Perot etalon consisting of the MQW layer 24 and p-type and n-type semitransparent mirrors 31 and 30 is changed. Thus, the wavelength of transmitted light is changed, and further, the optical filter is integrated and miniaturized.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical filter in the field of optical fiber communication, optical signal processing, or optical measurement.

(Prior Art) Wavelength separation on the receiving side of a wavelength division multiplexing optical transmission system has so far been achieved using interference filters shown in FIGS. 9(a) and (b) or a diffraction grating type demultiplexer shown in FIG. 12. is used.

The above-mentioned interference filter is made of a dielectric multi-N film in which a high refractive index telephone film and a low refractive index telephone film having an optical thickness close to λ/4 or λ/2 (λ: wavelength) are laminated alternately. It is made of a film and exhibits wavelength selectivity for specific wavelengths.

Figures 9(a) and 9(b) are cross-sectional views of a typical interference filter with two cavity layers, showing the detailed structure of a bandpass type interference filter coated with a full dielectric multilayer film. ing. Symbol H is a high refractive index material (
A layer coated with an optical thickness exactly at the storm wavelength (usually Zn5) is coated with a low refractive index material (usually NasAIFg).
) means a layer coated with an optical thickness of exactly the genus wavelength. The wavelength of "A wavelength" refers to the wavelength of peak transmittance. Each coating layer is coated using a vacuum deposition method. An aluminum ring protects the edges of the filter, and an epoxy adhesive layer is provided to protect the coating layer from moisture and to laminate the bandpass and blocking sections.

TlO2 is a high refractive index film material in a dielectric multilayer film.
and znS, and low refractive index film materials include S, O,
and Al2O3 are used. The wavelength selection characteristics largely depend on the number of layers of the dielectric film, the film material, etc., but recently S, 02 (low refractive index material, n = 1.46) and T
By laminating 20 to 40 layers of lO2 (high refractive index material, n = 2.3) using electron beam evaporation, band pass type (BPF), long wavelength band pass type (LWPF), short wavelength band pass type ( Various filter characteristics such as SWPF) have been realized (see FIG. 10). As illustrated in FIG. 10, the dielectric multilayer interference filter has a wavelength range other than the passband as a reflection range. Therefore, by arranging this at an angle, transmitted light and reflected light can be extracted at the same time.

The optical demultiplexer configuration shown in FIG. 11 is realized by utilizing this principle and arranging filters in multiple stages according to the multiplicity (however, in this case, the filter transmission center wavelengths are different). As shown in the figure, lenses (1
), (2) Interference film filter between (3), (4)
are arranged in a - column, and the filter (3),
(4) and this filter (3), (4)
The structure is such that the reflected light from the lens passes through the lenses (5) and (6), respectively. Also, interference film filter (3
) reflects light of wavelength λ, and filter (4) reflects light of wavelength λ.
The lens (1) has the property of reflecting light of 2.
) at wavelength λ1. λ2. When light of wavelength λ is incident, first, the light of wavelength λ is reflected by the filter (3) and exits from the lens (5), then the light of wavelength λ2 is reflected by the filter (4) and exits from the lens (6). ), which causes the filter (4
) only the light with wavelength λ3 passes through the lens (
2) emits light with the wavelength λ3.

Thus the wavelength λ1. λ2. It is now possible to split the light into λ3 light.

On the other hand, a typical diffraction grating type duplexer is a duplexer in which a concave diffraction grating (12) is attached to the output end of a two-dimensional waveguide (11) as shown in FIG. In the waveguide section (13), the longitudinal light confinement thickness is 10
On the other hand, in the lateral direction, a cylindrical concave diffraction grating (1
In step 2), the light is focused according to the wavelength. This diffraction grating (12)
is fabricated by selective etching of silicon.

FIG. 13 shows the spectral characteristics when the input fiber is a single mode optical fiber with a core diameter of 10 μm and the output fiber is a multimode fiber with a core diameter of 50/125 μm.

[Problem to be solved by the invention]

However, for the interference filter mentioned above, (transmission)
The problem is that the wavelength is not variable, and many types of filters with different center wavelengths are required depending on the degree of multiplication, resulting in a complicated configuration and being unsuitable for integration with light receiving/emitting elements, optical switches, etc.

On the other hand, an optical demultiplexer using a concave diffraction grating can provide an optical demultiplexer with a large degree of multiplicity, but the shape is relatively large and
It is expensive, difficult to use for two-way communication due to its structure, and is not suitable for integration with light-receiving and light-receiving elements.

The present invention was made to solve this problem, and an object of the present invention is to obtain an optical filter that can make the wavelength of transmitted light variable, and that can be integrated and miniaturized.

[Means to solve the problem]

The optical filter according to the present invention includes a buffer layer provided with a first electrode and a semiconductor substrate, a multiple quantum well layer laminated on the buffer layer, and a second electrode laminated on the multiple quantum well layer. a buffer layer in a light transmission path formed by each of the layers, and a semi-transparent mirror provided on the outer surface of the cap layer; The structure is such that a reverse bias voltage is applied.

[Effect]

The optical filter in this invention has a so-called Fabry Bellows resonator (or etalon) structure in which a nonlinear medium is sandwiched between semi-transparent mirrors, so it functions as an optical filter that transmits and reflects light of a predetermined wavelength, and also functions as a multi-quantum When an electric field is applied to the well layer, the equipolarized refractive index changes, so the resonance wavelength changes.

(Embodiment) In order to realize a wavelength tunable filter that is small in size, low in cost, and capable of being multiplied with other optical devices or electronic devices, an n4-buffer layer, an n4-buffer layer,
A semiconductor crystal in which a multi-quantum well layer (MQW layer) and a Pl-cap layer are sequentially grown, electrodes for applying a reverse bias voltage formed on both sides of the semiconductor crystal, and a material for forming a Fabry bellows resonator (or etalon). It is characterized by being configured with a semi-transparent mirror.

FIGS. 1(a) to (C) show a first embodiment of the present invention, and as shown in FIG. 1(b), a semiconductor substrate (22
) (for example, an N4-substrate), a buffer layer (
21) (for example, an n+- buffer layer) is laminated, and an n electrode (23) as a first electrode is laminated on the lower surface side of this n1-substrate (22). n1 buffer layer (
21) A multiple quantum well layer (hereinafter referred to as MQW layer) (24) is laminated on the upper surface side, and this MQW layer (24)
On the top side of the cap layer (for example, P2 cap layer) (2
5) is laminated, and furthermore, this P" cap layer (25)
A P electrode (26) serving as a second electrode is laminated on the upper surface side. (27) is the human light incident on this optical filter, and the light incident from above is transmitted to each layer (25), (24).
, (21) and is emitted downward as an emitted light (29) after passing through a light transmission path (28) formed by the light transmission path (28).

A semi-transparent mirror (30) such as an Au/S, N mirror is provided on the outer surface of the no-buffer layer (21) in the light transmission path (28), and similarly a Po-cap layer (25) in the light transmission path (28) is provided. ) is provided with a semi-transparent mirror (31) such as an AU/S or N mirror. In addition, each electrode (23),
(26), each of the above layers (25), (24),
(21) A reverse bias voltage can be applied to the PIN structure formed by (21). The semi-transparent mirror above (
31) is arranged in the opening (32) for light passage formed in the P electrode (26), and the semi-transparent mirror (30) is arranged in the N electrode (23) and the n1 substrate (22). The opening (33) is provided for the passage of light. Figure 1 (a
) is the energy band diagram of the multiple quantum well layer (24),
(41) is the quantum well layer, (42) is the barrier layer, and E
C represents the energy at the edge of the conduction band, and %'EV represents the energy at the edge of the valence band. Figure 1(c) shows the MQW layer (
24), and has a multilayer structure in which, for example, quantum well layers (41) of 100λ or less and barrier layers (42) of 100λ or less are alternately provided.

As shown in FIGS. 2 and 3, the basic configuration of this invention is to connect a nonlinear medium (51) (thickness L) to a pair of semi-transparent mirrors (
This is a so-called Fapley bellows resonator (or etalon) structure sandwiched between (52) and (52), and its effect will be explained. The transmittance T when light enters this resonator from one direction is as follows. Here, T , , F , and φ are determined by the reflectance of the mirror, absorption within the resonator, etc. Further, β is the wave number within the resonator. That is, when the refractive index within the resonator is n and the wavelength of light in vacuum is λ, β=2πn/λ (2). Assuming φ=0 for simplicity, βL=mπ(m
: integer) The transmittance T takes a peak value (resonance condition).

Therefore, if the refractive index n can be changed, the wavelength of light to be transmitted can be selected.

Next, the principle of refractive index change due to the quantum well structure will be explained. Figure 4(a) shows a normal quantum well structure (Flat
Figure 4 (b) shows an energy band diagram when an electric field is applied to a Gap Quantum Well (FGQW).
) is a quantum well structure (Grade
d shows an energy band diagram when an electric field is applied to the Gap Quantum Wall (GGQW). Same figure (
The broken line in b) shows the case when no electric field is applied, and the solid line shows the case when a positive electric field (the direction in the figure is taken in the positive direction) is applied. Figures 5 and 6 also show FGI and GGQ.
This is a calculation of the refractive index change due to the electric field near the absorption edge of W. Although the case of a ^1GaAs/GaAs-based quantum well structure is shown here, other materials such as InGaAs (P)/InP-based materials are also similar in terms of wood quality. FIG. 5 shows that the composition of the quantum well layer is GaAs.
So, the composition of the quantum well barrier layer (barrier layer) is ^lx Ga
+-1lAs(X-0,6), quantum well width Lz is 1z
= 100λ, refractive index change Δn due to electric field in FGQW (electric field strength is F = 40kV/c+n and 8
0 kV/cm). The arrow indicates H at electric field strength F = 80 kV/cm from the low energy side.
The energy levels of HI-El exciton and LHI-El exciton are shown, and the dashed line is the HI3-El exciton transition (forbidden transition).
The calculation results are shown ignoring the effect of . FIG. 6 shows that the composition of the quantum well layer is A1. xGa+-, As (x=O-0,
15), the composition of the quantum well barrier layer is ^lo, 6Gm0.
4AS, when quantum well width L2 is Lz = 100λ,
The refractive index change Δn due to the electric field in GGQW (the electric field is F
= ±60 kV/cn).

The arrow indicates electric field F = -80kV/c from the low energy side.
) I) Shows the energy levels of 11-El exciton and LHI-El exciton at m. In addition, HHI-El is an exciton (
(exciton) transition, and Ll (1-El represents the exciton transition from the light hole first quantum level to the electron first quantum level. Figures 5 and 6 above show the AlGaAs/Ga
Although the calculation results are for an As-based quantum well structure, the wood quality is the same for other material systems, such as the InGaAs (P) /InP system.

Next, the process for crystal growth and element fabrication of the optical filter shown in FIG. 1 is performed after H1. First, by metal organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE), an n+- buffer layer (21), an MQW layer (24), and Then, a P+- cap layer (25) is sequentially grown. Next, the back side (lower side in the figure) of the n'' substrate (22) is mirror-polished to make it thinner, and then an n electrode (23) for applying a reverse bias voltage is attached to the area excluding the area for light extraction. At the same time, a P electrode (26) for applying a reverse bias voltage is also formed on the surface side (top side in the figure) of the Pl-cap layer (25), excluding the area for light intake.

In order to form the opening (33) of the light extraction region, n
2. From the back side (lower side in the figure) of the board (22),
22) and the n+- buffer layer (21) in a cylindrical shape. As shown in FIG. 1(b), semi-transparent mirrors (for example, semi-transparent films) (31), (:+o) are formed on the surfaces of the light intake region and the light extraction region.

Therefore, the operating principle is that the n-electrode (23) and the p-electrode (23)
(2B) When a reverse bias voltage is applied between
Since an electric field is applied to the layer (24), the equipolar refractive index changes. Therefore, the resonant wavelength (wavelength of transmitted light) of the Fabry bellow etalon (or resonator) formed by this MQW layer (24) and the P-side and n-side semi-transparent mirrors (31) and (30) changes. I will do it. Next, to give an example of the material system used, the incident light (27)
If the wavelength of is in the long wavelength range, the following cases may apply.

(1) ---n' one substrate (22): InP (
2) ・n”-buffer layer (21): InP (
3)...MQW layer (24) - Quantum well layer (41): InGaAs or InGa
AsP/barrier layer (42): InP (4)/P''-cap layer (25): InP On the other hand, when the wavelength of the incident light (27) is short, the following may occur.

(1) =・n ”One substrate (22): GaAS (
2)'...n''-buffer layer (21): AIG
aAs (3)...MQW layer (24) ・Quantum well layer (41): GaAs ・Barrier layer (42): AIGaAs (4) ・P
”-Cap layer (25): AlGaAs Therefore, in this embodiment, the transmitted light wavelength is variable, integration with optical devices and electronic devices is possible, two-dimensional arraying can be easily realized, and wavelength multiplexing and demultiplexing light can be realized. It is ideal as a key device for circuits, etc. Furthermore, the size of one chip is approximately several hundred μm square, which is extremely small, and has the excellent effect that optical axis adjustment is not so severe.

FIG. 7 shows a second embodiment of the present invention, in which an n+- buffer layer (61) is provided on the upper surface of a semi-insulating semiconductor substrate (62), and the n0 buffer layer (
An n-electrode (63) is provided on the upper surface side of the lateral protrusion 61).

FIG. 8 shows a third embodiment of the present invention, and FIG.
This shows a case in which the configurations shown in the embodiments are arranged in a row. Therefore, the cross section taken along the line XX in the figure has the same configuration as that in FIG. 7 above.

Note that in each of the above embodiments, the light transmission direction may be reversed.

(Effects of the Invention) As explained above, the present invention has an optical filter having a Fabry-Perot resonator structure including a multiple quantum well layer, so that when an electric field is applied to the multiple quantum well layer, the equipolarized refractive index changes and the light is transmitted. This has the effect that the wavelength of light can be changed, and that the optical filter can be integrated and miniaturized.

[Brief explanation of drawings]

FIG. 1 is a sectional view of an optical filter showing an embodiment of the present invention, FIG. 2 is a perspective view for explaining the invention in detail,
Figure 3 is a front view, Figure 4 (a) is an energy band diagram when an electric field is applied to FGQW, Figure 4 (b) is an energy band diagram when an electric field is applied to GGQW,
5 and 6 are graphs showing spectra of changes in refractive index due to an electric field near the absorption edge of FGQW and GGQW, respectively; FIG. 7 is a cross-sectional view showing a second embodiment of the present invention; and FIG. is a perspective view showing the third embodiment, FIG. 9 is a sectional view of a conventional interference filter, FIG. 10 is a characteristic diagram of the interference filter, and FIG. 11 is a configuration diagram of an optical demultiplexer using the filter. FIG. 12 is a perspective view of a diffraction grating type duplexer, and FIG. 13 is a characteristic diagram of the duplexer. 21) , (at)...Buffer layer (n+- buffer layer) 22)...Semiconductor substrate (n-substrate) 23)
, (63)...first electrode (n electrode) 24)...
Multiple quantum well layer 25... Cap layer (Po-cap layer) 26)...
・Second electrode (P electrode) 28...Light transmission path (30), (31)...Semi-transparent mirror (δ2)...Semiconductor substrate (semi-insulating semiconductor substrate) Note that the same reference numerals are used in each figure. indicates the same or equivalent part. fig. F□ fig.

Claims (1)

[Claims] a buffer layer provided with a first electrode and a semiconductor substrate;
A multiple quantum well layer laminated on this buffer layer, a cap layer laminated on this multiple quantum well layer and provided with a second electrode, and a buffer layer and a cap layer in a light transmission path formed by each of the above layers. and a semi-transparent mirror provided on the outer surface of the optical filter, wherein a voltage is applied to each of the electrodes to provide a reverse bias to the PIN structure formed of each of the growth layers.