JPH01219717A - Optical modulator - Google Patents

Abstract

PURPOSE:To obtain the high-gain optical modulator which has a small spread spectrum and a wide modulation band at the time of modulation by performing n-type doping for the active layer of an LD optical modulator which is made of a semiconductor material and setting the composition wavelength of the active layer on a longer wavelength side than the wavelength of light to be modulated. CONSTITUTION:A semiconductor laser light amplifier is used as the LD optical modulator by modulating an implanted current and the wavelength lambdaS of the light to be modulated is 1.55mum. This optical modulator is constituted by forming an n-InP buffer layer 2, an InGaAsP active layer 3, an InGaAsP attachment back layer 4, and a p-InP clad layer 5 on an n-InP substrate 1 successively by a liquid crystal wavelength method and thus forming a LDH wafer. The band gap wavelength lambdag of this active layer 3 is set to 1.59-1.61mum, preferably, 1.6mum and the n-type doping is carried out by using Te. Thus, the high-gain optical modulator is realized which has the small spread spectrum and wide modulation band at the time of the modulation.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an optical modulator used in fields such as optical communication and optical switching.

(Prior Art) Optical modulators are important devices not only in ultrahigh-speed communication systems but also in systems such as optical switching and optical subscriber systems. Up until now, the optical modulators have been full, Ti on 1Nb03 substrate.
Directional coupler type Matsuhaka-Zehnder (M,Z) interferometric optical modulator using a diffusion waveguide (2) Utilizing the shift of the absorption edge due to the electric field of a bulk semiconductor, multiple quantum well (MQW) structure Electroabsorption optical modulators are known. (Magazine [I.E.E. Transactions on
Microuniven Theory and Techniques (I
EEETransaction on Microwa
ve Theory and Techniques)
J, Volume MTT-30, pp. 112)-1137, 1
982: Magazine [Electronics Letters (Ele
ctronics Letters>J-Volume 2), 6
Pages 93-694, 1985> Of these, (1) has the advantage of high-speed modulation and small spectral spread during modulation, but has the problem of relatively long element length and large polarization dependence of the incident light. . Also, although it is relatively small,
There is insertion loss. Although (2) is compact and capable of high-speed modulation, it has a drawback that the overall coupling loss is large because the absorption loss of light in the element is large and the coupling loss with the optical fiber is also large. In this way, conventionally known optical modulators
Insertion loss was a major problem.

Injecting a semiconductor laser optical amplifier to solve this point,
The applicant and others have proposed using the device as an optical modulator by modulating current. (48th Japan Society of Applied Physics Academic Conference Proceedings, Volume 3, 722p Paper No. 18p-ZG-6, 1987) In principle, this type of optical modulator (hereinafter abbreviated as LD optical modulator) Since it has a gain effect, it is possible to obtain gain along with the modulation operation.

Furthermore, the element length is as small as several 100 μm, and there is an advantage that dependence on incident polarization can be reduced due to the equipotency ratio of the waveguide structure.

(Problem to be solved by the invention) However, although the LD optical modulators reported so far have achieved internal gain, when a single mode fiber (SMF) is connected to the input and output, the gain is approximately 2 dB. It was a loss. Furthermore, the α parameter, which determines the spectrum spread during modulation, is very large, about 7, and the modulation band is also low.

An object of the present invention is to eliminate the above-mentioned problems and provide an LD optical modulator with high gain, small spectrum spread during modulation, and wide modulation band.

(Means for Solving the Problem) An optical modulator according to the present invention includes means for suppressing reflection from an active layer and an end face made of a semiconductor material, means for injecting modulated light into the active layer, and a modulated signal to the active layer. and a means for injecting a current according to the wavelength of the modulated light, wherein the compositional wavelength of the active layer is set to a longer wavelength side than the wavelength of the modulated light. be.

More preferably, in the optical modulator according to the present invention, the active layer is n
Characterized by being doped.

As the injection current increases, the gain spectrum of the semiconductor laser optical amplifier increases as the peak shifts to the shorter wavelength side due to the band filling effect. Figure 3 (a>
shows the relative positional relationship between λ5 and the gain spectrum when the modulated light wavelength λ5 and the active layer composition wavelength ^8 are approximately equal. As shown in the figure, in the high current injection state, there is a large deviation between λ5 and the gain peak wavelength, and sufficient gain cannot be obtained. Also,
It is known that the size of the α parameter, which determines the spectral spread during modulation, is wavelength dependent, and takes a large value for wavelengths on the longer wavelength side than the gain peak. (magazine[
Applied Physics Letters
Physics LetLers), Volume 42, 631
(Page 633, 1983) Therefore, if ^S and ^8 are made almost the same, the α parameter will also be used in a large state, which poses a problem.

To solve this problem, as shown in FIG. 3(b), ^8 may be set in advance on the longer wavelength side with respect to ^S.

By setting in this way, it is possible to increase the gain and suppress spectrum broadening during modulation.

On the other hand, the upper limit of the modulation band of the LD optical modulator is limited by the carrier lifetime. It is known that carrier lifetime is reduced by increasing the amount of doping into the active layer. Therefore, by doping the active layer of the LD optical modulator, the modulation band can be improved. In fact, in LEDs, 2 Gb/S NRZ modulation has been realized by p-type doping of the active layer. ([Electronics Letters (E l
Electronics Letters) J, Vol. 23, pp. 636-637, 1987) In the case of an LD optical amplifier, it is known that n-type doping increases the saturation optical output intensity. (Journal of the Institute of Electronics and Communication Engineers, No. J6
9-C, pp. 42)-431, 1986) In the case of LD optical modulators as well, increasing the saturation optical output intensity is important.
As for the doping type, n-type is excellent.

(Examples) The present invention will be explained in detail below using examples. 1st
The figure shows a perspective view of a light modulator according to the invention. Here, an example will be described in which an optical modulator for light with a wavelength λ51.55 μm is realized. First, manufacturing of the optical modulator according to the present invention will be explained. (100)n-1n
n-1nP buffer layer 2 on P substrate 1, InGa
An AsP active layer 3, an InGaAsP anti-meltback layer 4, and a p-1nP cladding layer 5 are successively grown by the liquid phase wavelength method to fabricate a DH wafer. InGaAsP active layer 3
The band gap wavelength λ8 of is 1.59 μm to 1.61 μm
m is preferably 1.6 μm, and using Te, 4×1
N-type doping of about 0.018 cta-3 was performed.

The thickness of the active layer 3 was 0.15 μm.

This DH wafer was coated with two n-1nP substrates 1 with a mesa 6 containing the active layer in between by photolithography.
Form to reach. Next, using a liquid phase growth method, p-]nP first current blocking layer 11, n-1nP second current blocking layer 12, p-1nP buried layer 13, p-
A 1nGaAsP cap layer 14 is sequentially grown.

To reduce element capacitance, two full-8
is also formed so as to reach the n-1nP substrate 1, and then the entire epitaxial layer of the wafer is coated with 5 layers using the CVD method.
An i02 film 9 is formed. Of this 5i02 film 9, only the upper part of the mesa 6 including the active layer is removed by etching.
A plFI ohmic electrode 10a is formed. In order to facilitate cleavage, the substrate is polished to a thickness of about 100 μm, and an n-type ohmic electrode 10b is also formed on the substrate side.

What about the wafers made in this way? Cleave on two sides to expose the plane perpendicular to the direction of 746.7.

Here, the element length was approximately 300 μm. A 5iNX anti-reflection coating film 15 is formed on the end face formed by this cleavage by plasma CVD, and the structure shown in FIG. 1 is completed.

Next, the operation of the optical modulator according to the present invention will be explained.

Figure 2 shows the configuration of the measurement system. Wavelength λ-1,553μm
The DFB-LD light 25 was coupled to the optical modulator 20 by the tip 5MF2)d, and the emitted light was taken out by the tip 5MF2)a to measure the characteristics. The input and output coupling losses are approximately 5 dB each. A DC bias from a DC power supply 22 and a high frequency signal bias T24 from a signal source 23 were applied to the electrodes of the optical modulator 20. First, when the optical modulator was driven with only the DC power supply 22 and operated as an optical amplifier,
A maximum gain of ~15 dB between SMFs was obtained.

Then, when a signal was applied from the signal source 23 and the modulation frequency band was measured, it was found to be 600 as a 3 dB lower frequency band.
MHz was obtained. In the sample of the non-doped active layer produced at the same time, the modulation band was 500 MHz, and it was confirmed that the modulation band was improved by doping. 600Mb/modulation with random pattern
Good eye patterns were observed with no equalization up to about S, and with one-stage equalization at t and 2 Gb/s.

The measured value of the α parameter was about 2 to 3, and the α parameter was reduced to less than 1/2 compared to the case where the signal light wavelength λS and the active layer band gap wavelength λ8 were almost the same (α~7). .

In the above embodiment, 1,55 is made of InGaAsP-based material.
Although we have described an example of realizing a μm band optical modulator, by changing the composition of the active layer, it is possible to
．． A 6 μm band optical modulator can be realized. It goes without saying that the present invention is applicable not only to this material system but also to a wide range of material systems having a gain mechanism.

(Effects of the Invention) As described above in detail, according to the present invention, an optical modulator with high gain, small spectrum spread during modulation, and wide modulation band can be realized.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the structure of an embodiment of the optical modulator according to the present invention, FIG. 2 is a diagram for explaining the operation of the optical modulator according to the present invention, and FIG. 3 is a diagram showing the difference between the gain spectrum and the signal light wavelength. FIG. In the figure, active layer, 10a, 10b are electrodes, 7.8 is a groove, 6 is a mesa, 9 is SiO□, 15 is SiN, 20 is an optical modulator, 2) a, 2) b are fibers, 22 is a DC power supply,
23 is a signal source, 24 is a bias T, and 25 is light.

Claims (2)

[Claims] (1) Optical modulation consisting of means for suppressing reflection from an active layer and end face made of a semiconductor material, means for injecting modulated light into the active layer, and means for injecting a current in accordance with a modulation signal into the active layer. An optical modulator, characterized in that the composition wavelength of the active layer is set to a longer wavelength side than the wavelength of the modulated light. (2) The optical modulator according to claim 1, wherein the active layer is n-type doped.