JPH0661578A - Distribution reflection device, waveguide type fabry-pero 't optical wavelength filter using the same, and semiconductor laser - Google Patents

Abstract

PURPOSE:To provide a semiconductor laser which oscillates at a single wavelength by adopting a distribution reflector which shows high reflectance in a wide wavelength region and a waveguide type Fabry-Pero't optical wavelength filter wherein it is used. CONSTITUTION:When current is made to flow to active waveguide regions 101, 104, laser oscillation is generated transmitting wavelength of a Fabry-pero't optical wavelength filter inside resonator is adjusted by making current flow to wavelength adjustment electrodes 9b, 9c provided to filter regions 102, 103, and oscillation wavelength is changed. Transmitting characteristic of the filter regions 102, 103 has a sharp peak periodically; however, a peak interval differs due to a difference of resonator length. In a structure wherein two filters are connected in series, only light of a wavelength lambda, wherein transmitting wevelength of both filters coincides is transmitted and the laser oscillates. Therefore, when current is made to flow through the electrode 9c on the region 103, refraction factor in the region reduces, transmitting wavelength shifts to the side of short wavelength and oscillation wavelength moves to lambda2, operational wavelength can be changed and a narrow band filter and a semiconductor laser can be acquired.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a waveguide type Fabry-Perot optical wavelength filter using a distributed reflector and a semiconductor, which are important for optical wavelength (frequency) multiplex communication systems and optical measurement in the optical communication field. It concerns lasers.

[0002]

2. Description of the Related Art In order to increase the amount of communication information in the future, research has been conducted on an optical wavelength (frequency) multiplex communication system that multiplexes a plurality of signal lights having different wavelengths and transmits them by one optical fiber. In the above-mentioned optical wavelength division multiplexing communication system, the optical wavelength filter is an important component for selecting only the signal light of a specific wavelength from the plurality of signal lights multiplexed in the wavelength region. As an optical wavelength filter, a Fabry-Perot interferometer type filter is often used in which two reflecting mirrors face each other in parallel at a constant interval and show sharp resonance characteristics due to multiple interference therebetween. In an optical communication system, since it is used in combination with a waveguide type optical component such as an optical fiber or a semiconductor laser, a waveguide type Fabry-Perot filter that can be easily optically coupled with the above waveguide type component has been studied. There is. Since the Fabry-Perot filter requires a high-reflectance mirror, a structure in which a high-reflection film is vapor-deposited on both end faces of the waveguide has been reported so far.

[0003]

However, in the above-mentioned conventional structure, since the waveguide must be cut, it is difficult to manufacture, and it is difficult to connect to another waveguide, resulting in a large connection loss. It was As a material that can obtain high reflectance without cutting the waveguide, a distributed reflector in which periodic unevenness is formed on the waveguide is known, but the distributed reflector is conventionally high only in a narrow wavelength range. Since it does not show reflectance,
It was not suitable for wavelength filters.

It is an object of the present invention to obtain a distributed reflector exhibiting a high reflectance in a wide wavelength range to form a Fabry-Perot optical wavelength filter and a semiconductor laser.

[0005]

The above-mentioned object is to provide a substrate,
An optical waveguide including an optical waveguide layer having an optical refractive index larger than that of the substrate and at least one optical confinement layer having a refractive index smaller than that of the optical waveguide layer. By forming periodical unevenness or periodical compositional change, the equivalent refractive index of the optical waveguide is periodically changed to form a diffraction grating.
In a distributed reflector having a reflecting action for light having a wavelength determined by the diffraction condition of gg, the length of one period of the unevenness is continuously or intermittently changed in the waveguide direction,
Alternatively, by arranging at least one phase shift having a minute difference in the length of one period, the Bragg wavelength of the diffraction grating is spatially changed so that high reflection can be obtained in a wide wavelength range. Forming reflectors on both sides of a flat optical waveguide that does not include a diffraction grating, and obtaining a waveguide type Fabry-Perot optical wavelength filter that exhibits sharp resonance characteristics due to Fabry-Perot interference between distributed reflectors. By including one or more of the above filters inside the resonator, a semiconductor laser that oscillates at a single wavelength is obtained.

[0006]

In the present invention, a diffraction grating in which the pitch of the diffraction grating changes continuously or intermittently is prepared as a distributed reflector exhibiting a high reflectance in a wide wavelength range.
Configure a Perot-type filter. 5A to 5C show examples of the diffraction grating according to the present invention, all of which have an optical waveguide structure. For example, light incident from the left end face of the figure propagates in the waveguide. Emit from the right end face. If the wavelength of the light matches the reflection wavelength of the diffraction grating,
Part or all of the incident light is reflected by the diffraction grating and emitted from the left end face. In the figure, (a) is an example in which the pitch of the diffraction grating is continuously changed, and the pitch linearly increases from 0.238 μm at the left end to 0.247 μm at the right end. Further, (b) is an example in which the pitch is intermittently changed, and the pitch is 0.238 μm at the left end.
To 0.247 μm at the right end from 0.001 μm in steps of 0.001 μm. Further, (c) is an example in which a plurality of phase shifts are formed and the pitch is equivalently changed. The pitch is constant at 0.242 μm, but one cycle length is short on the left side of the diffraction grating. Regarding the phase shift, on the right side, negative phase shifts having a long one cycle are formed at several places, and the intervals of the phase shifts are narrowed toward both ends in inverse proportion to the distance from the center.

FIG. 6 is a calculation example showing the reflectance of the diffraction grating as a function of the wavelength of light in the example shown in FIG. 5 (b). The length of the diffraction grating is 140 μm, and the effective refractive index (n _eq ) of the waveguide is 3.2025. In a normal diffraction grating, the peak wavelength of the reflectance is λ = 2 with respect to the pitch Λ.
It is given by n _eq Λ. The diffraction grating according to the present invention is also calculated from the center pitch of 0.242 μm by the above formula to be 1.55.
Although it exhibits a high reflectance centering on μm, the bandwidth exhibiting a high reflectance is significantly wider than that of an ordinary diffraction grating. The reflectance is 80% or more in the wavelength range of 1.52 μm to 1.58 μm, indicating good reflection characteristics. In the example shown in FIG. 6, the bandwidth is about 0.08 μm in full width at half maximum,
The band of a normal diffraction grating under the same conditions is about 0.001
While it is μm, it spreads about 8 times.

[0008]

Embodiments of the present invention will now be described with reference to the drawings. FIG. 1 is a first embodiment showing a waveguide type Fabry-Perot optical wavelength filter according to the present invention, FIG. 2 is a diagram showing transmission characteristics of the Fabry-Perot optical wavelength filter, and FIG. 3 is a semiconductor laser according to the present invention. Second Embodiment FIG. 4 and FIG. 4 are views showing the operation principle of the semiconductor laser, and FIG.
2 is a diagram showing the transmission characteristics of the area 02, and FIG. 7B is a diagram showing the transmission characteristics of the area of the filter 103.

The first embodiment shown in FIG. 1 is a waveguide type Fabry-Perot optical wavelength filter using the distributed reflector of the present invention described in claim 1. (A) is a structural diagram of the optical wavelength filter, and (b) is a diagram showing a diffraction grating. Figure 1 (a)
1, 1 is an n-type InP substrate, 3 is an InGaAsP inactive waveguide layer having a bandgap wavelength of 1.3 μm, 4 is a p-type InP cladding layer, and 5 is a p (+)-type InGaAsP
Cap layer 6, 6 p-type InP current blocking layer, 7 n-type InP current blocking layer, 8 n-type electrode, 9 p-type electrode,
Reference numeral 10 is a diffraction grating based on the distributed reflector according to the present invention described in the preceding paragraph. The diffraction grating 10 has a length of 140 μm as described in the previous section, and the diffraction grating is formed on both sides of the flat waveguide 11. The flat waveguide 11
Has a length of 10 μm.

A method of manufacturing the waveguide type Fabry-Perot optical wavelength filter of the first embodiment will be briefly described. First, n-type InP is formed by using a metalorganic vapor phase epitaxial growth method.
The inactive waveguide layer 3 is formed on the substrate 1. After that, the pattern of the diffraction grating whose pitch is modulated is transferred to the resist coated on the surface of the inactive waveguide layer 3 by using the electron beam exposure method, and the diffraction pattern 10 is etched by using the transferred pattern as a mask to form the diffraction grating 10. Form. A part without a diffraction grating is formed in a part of the transfer pattern at this time, and at the same time, the flat waveguide 11 is formed. Next, the waveguide is processed in a stripe shape to control the transverse mode, and the p-type InP current blocking layer 6, the n-type InP current blocking layer 7, and the p-type InP are again formed by using the metalorganic vapor phase epitaxial growth method.
The clad layer 4 and the p (+) type InGaAsP cap layer 5 are sequentially manufactured. Then, the p-type electrode 8 and the n-type electrode 9 are formed.

The diffraction gratings provided on both sides of the flat waveguide 11 form a distributed reflector by intermittently changing the pitch Λ as shown in FIG. 1 (b). 5
Since the reflectance is high in the wavelength range of 2 μm to 1.58 μm, the incident light undergoes multiple reflection between the distributed reflectors on both sides and reciprocates between the diffraction gratings 10 to cause so-called Fabry-Perot resonance. Figure 2 shows the transmission characteristics of the Fabry-Perot resonator. Transmission characteristics are about 2.5nm (0.0025μ
m) has sharp peaks at intervals, and each peak exhibits steep filter characteristics with a half value of 0.1 nm or less. Since this filter can be configured without cutting the optical waveguide, it can be easily connected in multiple stages and can be easily integrated with a semiconductor laser, an optical amplifier, a waveguide type photodetector, or the like. Furthermore, the wavelength interval of the transmittance peak and the central wavelength can be easily set by the length of the waveguide, and in this embodiment, fine adjustment can be performed by the change in the refractive index caused by the current injection from the electrode.

By connecting Fabry-Perot optical wavelength filters having different peak intervals in multiple stages, only one can be selected from the peaks shown in FIG.
A filter having a bandwidth of 0.1 nm or less between 1.58 μm can be formed.

FIG. 3 shows a second embodiment of the present invention as claimed in claim 3.
2 shows the structure of a semiconductor laser based on In FIG. 3, 1
Is an n-type InP substrate, 2 has a bandgap wavelength of 1.55
μm InGaAsP active layer, 3 InGaAsP optical confinement layer with bandgap wavelength of 1.3 μm, 4 p-type InP clad layer, 5 p (+) InGaAsP cap layer, 6 p-type InP current blocking layer, 7 n-type InP
Current blocking layer, 8 is n-type electrode, 9a is active waveguide region 1
P-type electrodes provided on 01 and 104, and 9b and 9c are p-type electrodes provided on the Fabry-Perot filter regions 102 and 103. The Fabry-Perot filter in the region 102 is the same as the filter shown in the first embodiment using the distributed reflector of the present invention, and the region 103 is the filter of the first embodiment in which the central flat waveguide portion is changed from 10 μm to 20 μm. It is an enlarged version.

A method of manufacturing the semiconductor laser with the wavelength sweeping function shown in the above embodiment will be briefly described. First, the n-type InP substrate 1 is formed by using the metalorganic vapor phase epitaxial growth method.
The active layer 2 and the light confinement layer 3 are formed on top. After that, a part of the active layer 2 is removed by using optical exposure and etching, and the removed portion is subjected to the same process as in the above embodiment.
The Perot filter regions 102 and 103 are formed. Then, in order to control the transverse mode, the waveguide is processed into a stripe shape, and again using the metalorganic vapor phase epitaxial growth method,
p-type InP current blocking layer 6, n-type InP current blocking layer 7, p-type InP cladding layer 4 and p (+)-type InG
The aAsP cap layer 5 is sequentially manufactured. After that, the p-type electrodes 9a to 9c and the n-type electrode 8 are formed, and further, in order to electrically separate the p-type electrodes 9a, 9b, and 9c from each other, the p-type electrode above the coupling portion of each region is formed. The electrode and the p (+) type InGaAsP cap layer 5 are removed.

In the semiconductor laser having the above-described structure, laser oscillation is generated by passing a current through the active waveguide regions 101 and 104, and the oscillation wavelength is the Fabry-
It is determined by the transmission wavelength of the Perot light wavelength filter.
The transmission wavelength of the filter is set to the filter regions 102 and 103.
The oscillation wavelength can be changed by applying a current to the wavelength adjusting electrodes 9b and 9c provided on the.

FIG. 4 shows the operating principle of the semiconductor laser, wherein (a) shows the transmission characteristics of the filter region 102 and (b) shows the transmission characteristics of the filter region 103. The transmission characteristics of the filter regions 102 and 103 both have sharp peaks periodically, but the peak intervals are 2.5 nm and 2.4 nm, respectively, due to the difference in resonator length. Therefore, in the structure of FIG. 3 in which two filters are connected in series,
Only light of a wavelength (λ ₁ ) where the transmission wavelengths of both filters match is transmitted, and the laser oscillates at this wavelength. When a current is applied to the electrode 9c on 103 in this state, the refractive index of this region decreases, and the transmission wavelength of the region 103 shifts to the short wavelength side and moves to the position shown by the broken line in the figure. Therefore, the wavelength λ ₁
The filter transmittance at is decreased and the oscillation wavelength is moved to λ ₂ . As described above, by supplying a current to one of the wavelength adjusting electrodes 9b and 9c, the oscillation wavelength can be largely changed at intervals of about 2.5 nm. On the other hand, when currents are simultaneously applied to the electrodes 9b and 9c, the transmission wavelengths of the two filters are simultaneously shifted to the short wavelength side, and the oscillation wavelength can be finely adjusted around λ ₁ .

[0017]

As described above, the distributed reflector according to the present invention and the waveguide type Fabry-Perot optical wavelength filter using the distributed reflector include an optical waveguide layer having an optical refractive index larger than that of the substrate, An optical waveguide including at least one optical confinement layer having a refractive index smaller than that of the optical waveguide layer, wherein periodical unevenness is formed or a periodic composition change is formed in one or more layers forming the optical waveguide. Thus, the equivalent refractive index of the optical waveguide is periodically changed to form a diffraction grating, and in the distributed reflector having the reflecting action for the light having the wavelength determined by the Bragg diffraction condition from the above period, By changing the length of one cycle of the unevenness continuously or intermittently in the waveguide direction, or by arranging at least one phase shift having a minute difference in the length of one cycle, the above diffraction Case Distributed reflectors with spatially varying Bragg wavelength are formed on both sides of a flat optical waveguide that does not include a diffraction grating, and a waveguide type Fabry-Perot optical wavelength filter is created by Fabry-Perot interference between the distributed reflectors. In addition, since one or more of the above filters are provided inside the resonator to form a semiconductor laser, the operating wavelength can be changed over the entire 1.55 μm band used in optical communication, and a narrow band filter and a semiconductor laser can be obtained. Obtainable.

[Brief description of drawings]

1A and 1B are diagrams of a first embodiment showing a waveguide type Fabry-Perot optical wavelength filter according to the present invention, FIG. 1A is a structural diagram of the filter, and FIG. 1B is a diagram showing a diffraction grating.

FIG. 2 is a diagram showing transmission characteristics of the Fabry-Perot optical wavelength filter.

FIG. 3 is a second embodiment diagram showing a semiconductor laser according to the present invention.

FIG. 4 is a diagram showing the operating principle of the semiconductor laser,
9A is a diagram showing a transmission characteristic of the filter region 102, and FIG. 9B is a diagram showing a transmission characteristic of the filter region 103.

FIG. 5 is a diagram showing the concept of a distributed reflector according to the present invention.

FIG. 6 is a diagram showing reflection characteristics of the distributed reflector shown in FIG. 5 (b).

[Explanation of symbols]

 1 substrate 2 optical waveguide layer 3 optical confinement layer 10 diffraction grating 11 flat optical waveguide not including diffraction grating

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hiroyuki Ishii 1-1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nihon Telegraph and Telephone Corporation

Claims (3)

[Claims] 1. An optical waveguide is formed on a substrate by an optical waveguide including an optical waveguide layer having an optical refractive index larger than that of the substrate and one or more optical confinement layers having a refractive index smaller than that of the optical waveguide layer. By forming periodic unevenness or periodic composition change in one or more layers, the equivalent refractive index of the optical waveguide is periodically changed to form a diffraction grating. In a distributed reflector having a reflection effect on light having a wavelength determined by Bragg's diffraction condition,
By changing the length of one cycle of the irregularities continuously or intermittently in the waveguide direction, or by arranging at least one phase shift having a minute difference in the length of one cycle, the diffraction A distributed reflector characterized in that the Bragg wavelength of a grating is spatially changed to obtain high reflection in a wide wavelength range. 2. The distributed reflector according to claim 1 is formed on both sides of a flat optical waveguide that does not include a diffraction grating, and the light transmittance is sharp due to Fabry-Perot interference between the distributed reflectors on both sides. A waveguide type Fabry-Perot optical wavelength filter characterized by exhibiting resonance characteristics. 3. The waveguide type fabric according to claim 2,
Including one or more Perot optical wavelength filters inside the resonator,
A semiconductor laser which oscillates at a single wavelength due to the wavelength selectivity of the optical wavelength filter.